Integrated Power Generation Using Molten Carbonate Fuel Cells

ABSTRACT

In various aspects, systems and methods are provided for integrated operation of molten carbonate fuel cells with turbines for power generation. Instead of selecting the operating conditions of a fuel cell to improve or maximize the electrical efficiency of the fuel cell, an excess of reformable fuel can be passed into the anode of the fuel cell to increase the chemical energy output of the fuel cell. The increased chemical energy output can be used for additional power generation, such as by providing fuel for a hydrogen turbine.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. Ser. No.14/207,700, filed Mar. 13, 2014, which is hereby incorporated byreference herein in its entirety and which itself claims the benefit ofprovisional U.S. Ser. Nos. 61/787,587, 61/787,697, 61/787,879, and61/788,628, all filed on Mar. 15, 2013, provisional U.S. Ser. Nos.61/884,376, 61/884,545, 61/884,565, 61/884,586, 61/884,605, and61/884,635, all filed on Sep. 30, 2013, and provisional U.S. Ser. No.61/889,757, filed on Oct. 11, 2013, each of which is incorporated byreference herein in its entirety.

This application is further related to co-pending U.S. Ser. Nos.14/197,397, 14/197,430, 14/197,551, and 14/197,613, all filed on Mar. 5,2014, and to co-pending U.S. Ser. Nos. 14/207,686, 14/207,687,14/207,688, 14/207,690, 14/207,691, 14/207,693, 14/207,696, 14/207,697,14/207,698, 14/697,699, 14/207,704, 14/207,705, 14/207,706, 14/207,708,14/207,710, 14/207,711, 14/207,712, 14/207,714, 14/207,721, 14/207,726,and 14/207,728, all filed on Mar. 13, 2014, each of which is herebyincorporated in its entirety by reference herein.

FIELD OF THE INVENTION

In various aspects, the invention is related to power productionprocesses integrated with use of molten carbonate fuel cells.

BACKGROUND OF THE INVENTION

Molten carbonate fuel cells utilize hydrogen and/or other fuels togenerate electricity. The hydrogen may be provided by reforming methaneor other reformable fuels in a steam reformer that is upstream of thefuel cell or within the fuel cell. Reformable fuels can encompasshydrocarbonaceous materials that can be reacted with steam and/or oxygenat elevated temperature and/or pressure to produce a gaseous productthat comprises hydrogen. Alternatively or additionally, fuel can bereformed in the anode cell in a molten carbonate fuel cell, which can beoperated to create conditions that are suitable for reforming fuels inthe anode. Alternately or additionally, the reforming can occur bothexternally and internally to the fuel cell.

Traditionally, molten carbonate fuel cells are operated to maximizeelectricity production per unit of fuel input, which may be referred toas the fuel cell's electrical efficiency. This maximization can be basedon the fuel cell alone or in conjunction with another power generationsystem. In order to achieve increased electrical production and tomanage the heat generation, fuel utilization within a fuel cell istypically maintained at 70% to 75%.

U.S. Published Patent Application 2011/0111315 describes a system andprocess for operating fuel cell systems with substantial hydrogencontent in the anode inlet stream. The technology in the '315publication is concerned with providing enough fuel in the anode inletso that sufficient fuel remains for the oxidation reaction as the fuelapproaches the anode exit. To ensure adequate fuel, the '315 publicationprovides fuel with a high concentration of H₂. The H₂ not utilized inthe oxidation reaction is recycled to the anode for use in the nextpass. On a single pass basis, the H₂ utilization may range from 10% to30%. The '315 reference does not describe significant reforming withinthe anode, instead relying primarily on external reforming.

U.S. Published Patent Application 2005/0123810 describes a system andmethod for co-production of hydrogen and electrical energy. Theco-production system comprises a fuel cell and a separation unit, whichis configured to receive the anode exhaust stream and separate hydrogen.A portion of the anode exhaust is also recycled to the anode inlet. Theoperating ranges given in the '810 publication appear to be based on asolid oxide fuel cell. Molten carbonate fuel cells are described as analternative.

U.S. Published Patent Application 2003/0008183 describes a system andmethod for co-production of hydrogen and electrical power. A fuel cellis mentioned as a general type of chemical converter for converting ahydrocarbon-type fuel to hydrogen. The fuel cell system also includes anexternal reformer and a high temperature fuel cell. An embodiment of thefuel cell system is described that has an electrical efficiency of about45% and a chemical production rate of about 25% resulting in a systemcoproduction efficiency of about 70%. The '183 publication does notappear to describe the electrical efficiency of the fuel cell inisolation from the system.

U.S. Pat. No. 5,084,362 describes a system for integrating a fuel cellwith a gasification system so that coal gas can be used as a fuel sourcefor the anode of the fuel cell. Hydrogen generated by the fuel cell isused as an input for a gasifier that is used to generate methane from acoal gas (or other coal) input. The methane from the gasifier is thenused as at least part of the input fuel to the fuel cell. Thus, at leasta portion of the hydrogen generated by the fuel cell is indirectlyrecycled to the fuel cell anode inlet in the form of the methanegenerated by the gasifier.

An article in the Journal of Fuel Cell Science and Technology (G.Manzolini et. al., J. Fuel Cell Sci. and Tech., Vol. 9, February 2012)describes a power generation system that combines a combustion powergenerator with molten carbonate fuel cells. Various arrangements of fuelcells and operating parameters are described. The combustion output fromthe combustion generator is used in part as the input for the cathode ofthe fuel cell. One goal of the simulations in the Manzolini article isto use the MCFC to separate CO₂ from the power generator's exhaust. Thesimulation described in the Manzolini article establishes a maximumoutlet temperature of 660° C. and notes that the inlet temperature mustbe sufficiently cooler to account for the temperature increase acrossthe fuel cell. The electrical efficiency (i.e. electricitygenerated/fuel input) for the MCFC fuel cell in a base model case is50%. The electrical efficiency in a test model case, which is optimizedfor CO₂ sequestration, is also 50%.

An article by Desideri et al. (Intl. J. of Hydrogen Energy, Vol. 37,2012) describes a method for modeling the performance of a powergeneration system using a fuel cell for CO₂ separation. Recirculation ofanode exhaust to the anode inlet and the cathode exhaust to the cathodeinlet are used to improve the performance of the fuel cell. The modelparameters describe an MCFC electrical efficiency of 50.3%.

SUMMARY OF THE INVENTION

In an aspect, a method for producing electricity is provided. The methodincludes introducing a fuel stream comprising a reformable fuel into theanode of a molten carbonate fuel cell, an internal reforming elementassociated with the anode, or a combination thereof; introducing acathode inlet stream comprising CO₂ and O₂ into the cathode of the fuelcell; generating electricity within the molten carbonate fuel cell, themolten carbonate fuel cell being operated at a fuel utilization of about60% or less; generating an anode exhaust comprising H₂, CO, and CO₂;separating, from at least a portion of the anode exhaust, a gas streamcomprising at least about 80 vol % H₂; and combusting at least a portionof the separated gas stream to produce electricity.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 schematically shows an example of a configuration for moltencarbonate fuel cells and associated reforming and separation stages.

FIG. 2 schematically shows another example of a configuration for moltencarbonate fuel cells and associated reforming and separation stages.

FIG. 3 schematically shows an example of the operation of a moltencarbonate fuel cell.

FIG. 4 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIG. 5 schematically shows an example of a combined cycle system forgenerating electricity based on combustion of a carbon-based fuel.

FIG. 6 schematically shows an example of a configuration for generatingelectricity.

FIG. 7 shows results of simulations of a system for generatingelectricity.

DETAILED DESCRIPTION OF THE EMBODIMENTS Overview

In various aspects, the operation of molten carbonate fuel cells can beintegrated with a variety of chemical and/or materials productionprocesses, including but not limited to providing a chemical output thatcan be used to generate further electrical power. For example, at leasta portion of the anode exhaust can be used as a fuel for a turbinepowered by combustion of hydrogen. Using an MCFC to provide hydrogen fora hydrogen turbine can allow for enhanced power generation, whileoptionally but advantageously reducing or minimizing the amount ofgreenhouse gases generated and/or exhausted to the environment.

Integration with a Hydrogen Turbine

In some aspects, an objective in producing low-carbon power can be toincrease or maximize the total power output while maintaining high CO₂capture efficiency and/or while efficiently utilizing existing systems.In a conventional system, a gas turbine can be connected to a MCFCsystem such that the gas turbine produces an exhaust stream comprisingCO₂ that serves as a component of the cathode inlet providing heat andCO₂ to the cathode. For this configuration, as is known in theliterature, CO₂ can be captured by conventional means and the MCFCsystem can be operated at relatively high fuel utilizations (typicallyabove 70% to about 80%, or about 75%) to maintain heat balance withinthe MCFC under ordinary operating conditions.

The efficient utilization of the MCFC can be improved by lowering thefuel utilization to process excess fuel, for example methane, andproduce excess syngas as an exhaust. This exhaust/effluent can undergovarious separations to yield a syngas stream that can be useful for avariety of chemical and industrial purposes. However, where syngas isnot useful as a feed stock, and/or for cases where electrical powergeneration can be a primary goal, generation of a syngas stream may notprovide additional low-carbon power.

In various aspects, systems and methods are provided for producing anincreased or maximized amount of electrical power from a fixed MCFCsystem while optionally but preferably providing for consistent highcarbon capture. In some aspects, such a system can be provided bycombining the use of a conventional gas turbine as the CO₂ source forthe MCFC cathode, a low fuel utilization for production of high amountsof syngas, and a combination of separation and/or conversion systemsthat can allow for increased production of hydrogen derived from theMCFC anode exhaust. This hydrogen stream derived from the anode exhaustcan then be introduced into a second hydrogen turbine where additionalpower can be generated with reduced or minimized emissions of CO₂.Because the second turbine can be powered by the hydrogen-containingstream derived from the anode exhaust, the amount of additional CO₂generated in order to power the second turbine can be limited to, forexample, carbon oxide(s) and carbon fuel residual components in thehydrogen-containing stream. Additionally or alternatively, the hydrogenfrom the anode exhaust can be used to generate electricity in othermanners, such as by combusting the hydrogen to raise steam, which canthen be used to generate electricity. Further additionally oralternately, a portion of the hydrogen derived from the anode exhaustcan be used as an input for the first (conventional) turbine. This canbe beneficial, for example, if the carbon-containing fuel for the firstturbine has an elevated content of inerts, such as CO₂ and/or N₂.

In addition to use within a refinery, hydrogen can more generally beuseful for a wide variety of products and processes, as it produces onlywater vapor on combustion. However, most conventional approaches tomaking hydrogen can require large emissions of carbon. For example,production of hydrogen from steam reforming of methane can produce CO₂(from the carbon in the methane) and waste heat. Production of hydrogenfrom electrolysis can require electricity, which is typically generatedfor the electrical grid based on combustion of a mixture of fossilfuels. These production systems can all typically result in effluentexhausts comprising CO₂. Applications such as fuel cell vehicles canrequire low-temperature fuel cells which utilize high purity hydrogen.While the vehicle does not produce much carbon emissions, the productionof the hydrogen for the vehicle can be inefficient, not easily adaptedto smaller-scale, and can produce significant carbon emissions.

In some additional or alternative aspects, the systems and methodsherein can facilitate separation of CO₂ efficiently from the process asan integral part of the separation and hydrogen purification steps. TheCO₂ can then be captured and/or used for other useful processes. Thiscan occur at high overall system efficiency, as compared to conventionalmeans of producing net hydrogen production/export, especially at smallscale and under variable load.

The use of a MCFC system for hydrogen production for use in subsequentprocesses that may generate electrical power and/or heat can allow forlow-emission energy production at high efficiency and with reduced orminimized carbon emissions. The MCFC system can dynamically respond tovarying needs for hydrogen by adjusting the ratio of chemical energyproduction to electrical energy production and can be suitable for useswhere loads and demands may not be constant—varying from heightenedelectrical production with little or no excess hydrogen to heightenedhydrogen production. Additionally, the integrated system can be scaledover a wider range of applications with high efficiency, as comparedwith larger scale systems such as methane steam reformers. This couldallow, for example, for co-production of hydrogen for other uses, suchas a fuel cell vehicle system, and for variable electrical power orsimply to vary electrical power output.

For example, in some operating configurations, the base gas turbine(such as a turbine powered by combustion of a carbon-containing fuel)can be run at constant high-efficiency conditions with the MCFC systemrun under variable fuel utilizations to yield different electrical andhydrogen production values, which can control the electrical output fromthe entire system. The amount of hydrogen and electricity can be varieddepending on overall demand while maintaining overall high systemefficiency. Hydrogen for export into boilers and/or other combined heatand power systems can allow for the constant production of electricity,e.g., in stand-alone generation, along with a variable amount ofcarbon-free heat via the production of hydrogen with subsequentcombustion in heater/boiler and/or similar systems. For example, aninstallation could produce primarily electricity in the summer for airconditioning systems while switching to a mix of primarily chemicalenergy in the winter for heating operations. During high electricaldemand, increased hydrogen can be sent to the hydrogen turbine formaximized electrical production. Other applications can include systemsdesigned to provide on-site hydrogen such as in laboratories and othertechnical and manufacturing facilities, where some hydrogen can beneeded along with a need for electrical energy.

In aspects involving hydrogen production and/or electrical powergeneration, the anode inlet can be fed by fresh methane, anothersuitable hydrocarbon fuel, and/or the combination of fresh fuel andrecycled CO and/or H₂ from the various processes. The anode outletstream comprising H₂ and/or CO can provide the components to producehydrogen. This is typically done through some combination of reaction,separation, and purification steps. An example can be a first stageemploying water-gas shift to shift as much as possible (nearly all) ofthe CO to H₂ by the reaction H₂O+CO ═H₂+CO₂, followed by a second (andpotentially subsequent) stage(s) that can remove H₂O and/or CO₂ from theH₂, and can provide a suitable purity product. Such stages can includePSA, cryogenic separation, membranes, and/or other known separationmethods, either individually or in combination. The off-gasses fromthese steps can be recycled and/or can be used to provide heat for inletstreams. The separated CO₂ can be used as a recycle stream and/or can becaptured and optionally used for other processes. The cathode inlet canbe composed of recycled CO₂ from the overall process and/or of CO₂produced by the combustion of fresh (or recycled) fuel used to provideheat to the inlet streams. In some preferred aspects, the cathode inletcan include at least a portion of a combustion exhaust from a firstconventional combustion turbine. The cathode effluent can typically beexhausted to the atmosphere, optionally but preferably after heatrecovery to, for example, provide heat for other process streams and/orin combined cycle electrical production, though the cathode effluentcould optionally but less preferably be sent for further treatment, ifdesired.

MCFC systems integrated into carbon-free heat and power applications canbe used over a range of operating conditions comprising high (e.g., fromabout 60% to about 70%) fuel utilization with low hydrogen make to lowfuel utilizations (e.g., from about 20% to about 60%) with increasedhydrogen production. Examples of low fuel utilizations can include atleast about 20%, such as at least about 30%, and/or about 60% or less,such as about 50% or less. The exact operational range of an individualapplication can vary widely both by application and over time. Theability to adapt to this operational range can be a desirable advantage.The number of separation stages and the purity achieved can depend onthe ultimate application. Simple production of hydrogen forlow-emissions heat can be tolerant to modest impurities in the hydrogen,as even a few percent CO₂ and/or CO in emitted streams could stillresult in very low overall carbon emissions. High purificationapplications such as fuel cell vehicles and/or hydrogen for laboratoriescan require multiple steps (e.g. cryogenic separation, followed by PSA)to achieve purity specifications.

In other configurations, the MCFC can be operated under lower fuelutilizations with the excess anode outlet fuel used to provide heatand/or power. In both cases, an advantage can be that the “base load”power output of the fixed gas turbine and MCFC systems can remainapproximately constant, while a small sub-section of the overallprocess, the hydrogen turbine, can be used for variable load. Thecombination of variable fuel utilization and variable hydrogen turbinefeed can allow the overall plant to meet a large variety of needs forheat, electricity, and/or hydrogen demand, while operating the majorsubsystems of the plant at fairly consistent operating conditions.

FIG. 6 shows an example flow sheet of an integrated power generationMCFC process that can produce low CO₂ emissions power from a convenientcarbon-containing fuel, such as natural gas and/or methane. In thisscheme a natural gas fired turbine 640 can combust oxidant (air) 601 andmethane/natural gas 602 to generate power and exhaust gas stream 603.Exhaust 603 can be fed into a cathode of MCFC 630. The anode of MCFC 630can be fed additional fuel (methane/natural gas) 605 and steam 606.Through an electrochemical reaction, the MCFC 630 can generate power,can produce a CO₂ depleted cathode exhaust 604, and can produce an anodeexhaust 607 that contains H₂/CO₂/CO. Heat can be recovered from cathodeexhaust 604, and then cathode exhaust 604 can be emitted to theatmosphere and/or subject to further treatment, if desired. Anodeexhaust 607 can be shifted in a water gas shift reactor 660 to increasethe H₂ concentration. The shift reactor effluent can undergo separations670 to remove water 608, recover CO₂ 609, and form a separated stream610 containing H₂. CO₂-containing stream 609 can be compressed topipeline conditions and can then be sold for use, used for a differentprocess, and/or sequestered. The separated stream 610 can be combinedwith oxidant (air) 611 and sent to a hydrogen turbine 650 to generateadditional power. The exhaust 612 from the hydrogen turbine 650 can bemostly water and N₂ and can be emitted to the atmosphere and/or subjectto further treatment, if desired.

As an example, simulations were performed using a configuration similarto the system shown in FIG. 6. Comparative simulations were alsoperformed for systems where a hydrogen turbine was not included. In thecomparative simulations, fuel (comprising hydrogen) from the anodeexhaust was instead recycled to the combustion zone for the conventionalturbine. It is noted that the size of the conventional turbine was heldconstant in the simulations, so recycle of fuel from the anode exhaustresulted in a reduction in the amount of fresh natural gas delivered tothe conventional turbine.

The results from the simulations are shown in FIG. 7. The results inFIG. 7 appeared to show that, at conventional fuel utilizations, such asa fuel utilization of ˜75%, use of a second hydrogen turbine may not beas beneficial. At a fuel utilization of ˜75%, the results appeared toshow that using the second hydrogen turbine can reduce the overallefficiency of generation of electrical power while also reducing the netpower produced.

For a fuel utilization of about 50%, the simulation results in FIG. 7appeared to show the benefits of operating with a second hydrogenturbine. The overall efficiency of the system with the second hydrogenturbine still appeared to be lower, as the overall efficiency was about50% versus the ˜55% that was simulated for the comparative example.However, the simulations appeared to show that operating at about 50%fuel utilization resulted in a larger amount of power produced (about624 MW) than any of the comparative examples, while also appearing tohave lower emissions of CO₂ per MWhr (about 144 lbs/MWhr) relative toany of the comparative examples. The simulations at a fuel utilizationof about 30% appeared to show even greater benefits of producing largehydrogen volumes by operating at lower fuel utilization. The simulationsappeared to show significantly increased power production (about 790 MW)while also significantly reducing the amount of CO₂ emissions (about 113lbs/MWhr). The combination of increased power and reduced CO₂ emissionsappeared to be achieved in the simulations in part due to the increasedamount of CO₂ captured (about 1.92 Mtons/yr at about 50% fuelutilization, about 2.56 Mtons/year at about 30% fuel utilization). As aresult, the simulations appeared to show that use of a hydrogen turbine,in combination with a fuel utilization of about 60% or less, such asabout 50% or less, can provide unexpectedly low CO₂ emissions per unitof energy generated while providing increased electrical energyproduction.

Additional Fuel Cell Operation Strategies

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell canbe operated so that the amount of reforming can be selected relative tothe amount of oxidation in order to achieve a desired thermal ratio forthe fuel cell. As used herein, the “thermal ratio” is defined as theheat produced by exothermic reactions in a fuel cell assembly divided bythe endothermic heat demand of reforming reactions occurring within thefuel cell assembly. Expressed mathematically, the thermal ratio(TH)=Q_(EX)/Q_(EN), where Q_(EX) is the sum of heat produced byexothermic reactions and Q_(EN) is the sum of heat consumed by theendothermic reactions occurring within the fuel cell. Note that the heatproduced by the exothermic reactions corresponds to any heat due toreforming reactions, water gas shift reactions, and the electrochemicalreactions in the cell. The heat generated by the electrochemicalreactions can be calculated based on the ideal electrochemical potentialof the fuel cell reaction across the electrolyte minus the actual outputvoltage of the fuel cell. For example, the ideal electrochemicalpotential of the reaction in a MCFC is believed to be about 1.04V basedon the net reaction that occurs in the cell. During operation of theMCFC, the cell will typically have an output voltage less than 1.04 Vdue to various losses. For example, a common output/operating voltagecan be about 0.7 V. The heat generated is equal to the electrochemicalpotential of the cell (i.e. ˜1.04V) minus the operating voltage. Forexample, the heat produced by the electrochemical reactions in the cellis ˜0.34 V when the output voltage of ˜0.7V. Thus, in this scenario, theelectrochemical reactions would produce ˜0.7 V of electricity and ˜0.34V of heat energy. In such an example, the ˜0.7 V of electrical energy isnot included as part of Q_(EX). In other words, heat energy is notelectrical energy.

In various aspects, a thermal ratio can be determined for any convenientfuel cell structure, such as a fuel cell stack, an individual fuel cellwithin a fuel cell stack, a fuel cell stack with an integrated reformingstage, a fuel cell stack with an integrated endothermic reaction stage,or a combination thereof. The thermal ratio may also be calculated fordifferent units within a fuel cell stack, such as an assembly of fuelcells or fuel cell stacks. For example, the thermal ratio may becalculated for a single anode within a single fuel cell, an anodesection within a fuel cell stack, or an anode section within a fuel cellstack along with integrated reforming stages and/or integratedendothermic reaction stage elements in sufficiently close proximity tothe anode section to be integrated from a heat integration standpoint.As used herein, “an anode section” comprises anodes within a fuel cellstack that share a common inlet or outlet manifold.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a thermal ratio. Where fuel cells are operatedto have a desired thermal ratio, a molten carbonate fuel cell can beoperated to have a thermal ratio of about 1.5 or less, for example about1.3 or less, or about 1.15 or less, or about 1.0 or less, or about 0.95or less, or about 0.90 or less, or about 0.85 or less, or about 0.80 orless, or about 0.75 or less. Additionally or alternately, the thermalratio can be at least about 0.25, or at least about 0.35, or at leastabout 0.45, or at least about 0.50. Additionally or alternately, in someaspects the fuel cell can be operated to have a temperature rise betweenanode input and anode output of about 40° C. or less, such as about 20°C. or less, or about 10° C. or less. Further additionally oralternately, the fuel cell can be operated to have an anode outlettemperature that is from about 10° C. lower to about 10° C. higher thanthe temperature of the anode inlet. Still further additionally oralternately, the fuel cell can be operated to have an anode inlettemperature that is greater than the anode outlet temperature, such asat least about 5° C. greater, or at least about 10° C. greater, or atleast about 20° C. greater, or at least about 25° C. greater. Yet stillfurther additionally or alternately, the fuel cell can be operated tohave an anode inlet temperature that is greater than the anode outlettemperature by about 100° C. or less, such as by about 80° C. or less,or about 60° C. or less, or about 50° C. or less, or about 40° C. orless, or about 30° C. or less, or about 20° C. or less.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated with increased productionof syngas (or hydrogen) while also reducing or minimizing the amount ofCO₂ exiting the fuel cell in the cathode exhaust stream. Syngas can be avaluable input for a variety of processes. In addition to having fuelvalue, syngas can be used as a raw material for forming other highervalue products, such as by using syngas as an input for Fischer-Tropschsynthesis and/or methanol synthesis processes. One option for makingsyngas can be to reform a hydrocarbon or hydrocarbon-like fuel, such asmethane or natural gas. For many types of industrial processes, a syngashaving a ratio of H₂ to CO of close to 2:1 (or even lower) can often bedesirable. A water gas shift reaction can be used to reduce the H₂ to COratio in a syngas if additional CO₂ is available, such as is produced inthe anodes.

One way of characterizing the overall benefit provided by integratingsyngas generation with use of molten carbonate fuel cells can be basedon a ratio of the net amount of syngas that exits the fuel cells in theanode exhaust relative to the amount of CO₂ that exits the fuel cells inthe cathode exhaust. This characterization measures the effectiveness ofproducing power with low emissions and high efficiency (both electricaland chemical). In this description, the net amount of syngas in an anodeexhaust is defined as the combined number of moles of H₂ and number ofmoles of CO present in the anode exhaust, offset by the amount of H₂ andCO present in the anode inlet. Because the ratio is based on the netamount of syngas in the anode exhaust, simply passing excess H₂ into theanode does not change the value of the ratio. However, H₂ and/or COgenerated due to reforming in the anode and/or in an internal reformingstage associated with the anode can lead to higher values of the ratio.Hydrogen oxidized in the anode can lower the ratio. It is noted that thewater gas shift reaction can exchange H₂ for CO, so the combined molesof H₂ and CO represents the total potential syngas in the anode exhaust,regardless of the eventual desired ratio of H₂ to CO in a syngas. Thesyngas content of the anode exhaust (H₂+CO) can then be compared withthe CO₂ content of the cathode exhaust. This can provide a type ofefficiency value that can also account for the amount of carbon capture.This can equivalently be expressed as an equation as

Ratio of net syngas in anode exhaust to cathode CO₂=net moles of(H₂+CO)_(ANODE)/moles of (CO₂)_(CATHODE)

In various aspects, the ratio of net moles of syngas in the anodeexhaust to the moles of CO₂ in the cathode exhaust can be at least about2.0, such as at least about 3.0, or at least about 4.0, or at leastabout 5.0. In some aspects, the ratio of net syngas in the anode exhaustto the amount of CO₂ in the cathode exhaust can be still higher, such asat least about 10.0, or at least about 15.0, or at least about 20.0.Ratio values of about 40.0 or less, such as about 30.0 or less, or about20.0 or less, can additionally or alternately be achieved. In aspectswhere the amount of CO₂ at the cathode inlet is about 6.0 volume % orless, such as about 5.0 volume % or less, ratio values of at least about1.5 may be sufficient/realistic. Such molar ratio values of net syngasin the anode exhaust to the amount of CO₂ in the cathode exhaust can begreater than the values for conventionally operated fuel cells.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated at a reduced fuelutilization value, such as a fuel utilization of about 50% or less,while also having a high CO₂ utilization value, such as at least about60%. In this type of configuration, the molten carbonate fuel cell canbe effective for carbon capture, as the CO₂ utilization canadvantageously be sufficiently high. Rather than attempting to maximizeelectrical efficiency, in this type of configuration the totalefficiency of the fuel cell can be improved or increased based on thecombined electrical and chemical efficiency. The chemical efficiency canbe based on withdrawal of a hydrogen and/or syngas stream from the anodeexhaust as an output for use in other processes. Even though theelectrical efficiency may be reduced relative to some conventionalconfigurations, making use of the chemical energy output in the anodeexhaust can allow for a desirable total efficiency for the fuel cell.

In various aspects, the fuel utilization in the fuel cell anode can beabout 50% or less, such as about 40% or less, or about 30% or less, orabout 25% or less, or about 20% or less. In various aspects, in order togenerate at least some electric power, the fuel utilization in the fuelcell can be at least about 5%, such as at least about 10%, or at leastabout 15%, or at least about 20%, or at least about 25%, or at leastabout 30%. Additionally or alternatively, the CO₂ utilization can be atleast about 60%, such as at least about 65%, or at least about 70%, orat least about 75%.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell canbe operated at conditions that increase or maximize syngas production,possibly at the detriment of electricity production and electricalefficiency. Instead of selecting the operating conditions of a fuel cellto improve or maximize the electrical efficiency of the fuel cell,operating conditions, possibly including an amount of reformable fuelpassed into the anode, can be established to increase the chemicalenergy output of the fuel cell. These operating conditions can result ina lower electrical efficiency of the fuel cell. Despite the reducedelectrical efficiency, optionally, but preferably, the operatingconditions can lead to an increase in the total efficiency of the fuelcell, which is based on the combined electrical efficiency and chemicalefficiency of the fuel cell. By increasing the ratio of reformable fuelintroduced into the anode to the fuel that is actually electrochemicallyoxidized at the anode, the chemical energy content in the anode outputcan be increased.

In some aspects, the reformable hydrogen content of reformable fuel inthe input stream delivered to the anode and/or to a reforming stageassociated with the anode can be at least about 50% greater than the netamount of hydrogen reacted at the anode, such as at least about 75%greater or at least about 100% greater. Additionally or alternately, thereformable hydrogen content of fuel in the input stream delivered to theanode and/or to a reforming stage associated with the anode can be atleast about 50% greater than the net amount of hydrogen reacted at theanode, such as at least about 75% greater or at least about 100%greater. In various aspects, a ratio of the reformable hydrogen contentof the reformable fuel in the fuel stream relative to an amount ofhydrogen reacted in the anode can be at least about 1.5:1, or at leastabout 2.0:1, or at least about 2.5:1, or at least about 3.0:1.Additionally or alternately, the ratio of reformable hydrogen content ofthe reformable fuel in the fuel stream relative to the amount ofhydrogen reacted in the anode can be about 20:1 or less, such as about15:1 or less or about 10:1 or less. In one aspect, it is contemplatedthat less than 100% of the reformable hydrogen content in the anodeinlet stream can be converted to hydrogen. For example, at least about80% of the reformable hydrogen content in an anode inlet stream can beconverted to hydrogen in the anode and/or in an associated reformingstage(s), such as at least about 85%, or at least about 90%.Additionally or alternately, the amount of reformable fuel delivered tothe anode can be characterized based on the Lower Heating Value (LHV) ofthe reformable fuel relative to the LHV of the hydrogen oxidized in theanode. This can be referred to as a reformable fuel surplus ratio. Invarious aspects, the reformable fuel surplus ratio can be at least about2.0, such as at least about 2.5, or at least about 3.0, or at leastabout 4.0. Additionally or alternately, the reformable fuel surplusratio can be about 25.0 or less, such as about 20.0 or less, or about15.0 or less, or about 10.0 or less.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can also be operated at conditions thatcan improve or optimize the combined electrical efficiency and chemicalefficiency of the fuel cell. Instead of selecting conventionalconditions for maximizing the electrical efficiency of a fuel cell, theoperating conditions can allow for output of excess synthesis gas and/orhydrogen in the anode exhaust of the fuel cell. The synthesis gas and/orhydrogen can then be used in a variety of applications, includingchemical synthesis processes and collection of hydrogen for use as a“clean” fuel. In aspects of the invention, electrical efficiency can bereduced to achieve a high overall efficiency, which includes a chemicalefficiency based on the chemical energy value of syngas and/or hydrogenproduced relative to the energy value of the fuel input for the fuelcell.

In some aspects, the operation of the fuel cells can be characterizedbased on electrical efficiency. Where fuel cells are operated to have alow electrical efficiency (EE), a molten carbonate fuel cell can beoperated to have an electrical efficiency of about 40% or less, forexample, about 35% EE or less, about 30% EE or less, about 25% EE orless, or about 20% EE or less, about 15% EE or less, or about 10% EE orless. Additionally or alternately, the EE can be at least about 5%, orat least about 10%, or at least about 15%, or at least about 20%.Further additionally or alternately, the operation of the fuel cells canbe characterized based on total fuel cell efficiency (TFCE), such as acombined electrical efficiency and chemical efficiency of the fuelcell(s). Where fuel cells are operated to have a high total fuel cellefficiency, a molten carbonate fuel cell can be operated to have a TFCE(and/or combined electrical efficiency and chemical efficiency) of about55% or more, for example, about 60% or more, or about 65% or more, orabout 70% or more, or about 75% or more, or about 80% or more, or about85% or more. It is noted that for a total fuel cell efficiency and/orcombined electrical efficiency and chemical efficiency, any additionalelectricity generated from use of excess heat generated by the fuel cellcan be excluded from the efficiency calculation.

In various aspects of the invention, the operation of the fuel cells canbe characterized based on a desired electrical efficiency of about 40%or less and a desired total fuel cell efficiency of about 55% or more.Where fuel cells are operated to have a desired electrical efficiencyand a desired total fuel cell efficiency, a molten carbonate fuel cellcan be operated to have an electrical efficiency of about 40% or lesswith a TFCE of about 55% or more, for example, about 35% EE or less withabout a TFCE of 60% or more, about 30% EE or less with about a TFCE ofabout 65% or more, about 25% EE or less with about a 70% TFCE or more,or about 20% EE or less with about a TFCE of 75% or more, about 15% EEor less with about a TFCE of 80% or more, or about 10% EE or less withabout a TFCE of about 85% or more.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated at conditions that canprovide increased power density. The power density of a fuel cellcorresponds to the actual operating voltage V_(A) multiplied by thecurrent density I. For a molten carbonate fuel cell operating at avoltage V_(A), the fuel cell also can tend to generate waste heat, thewaste heat defined as (V₀−V_(A))*I based on the differential betweenV_(A) and the ideal voltage V₀ for a fuel cell providing current densityI. A portion of this waste heat can be consumed by reforming of areformable fuel within the anode of the fuel cell. The remaining portionof this waste heat can be absorbed by the surrounding fuel cellstructures and gas flows, resulting in a temperature differential acrossthe fuel cell. Under conventional operating conditions, the powerdensity of a fuel cell can be limited based on the amount of waste heatthat the fuel cell can tolerate without compromising the integrity ofthe fuel cell.

In various aspects, the amount of waste heat that a fuel cell cantolerate can be increased by performing an effective amount of anendothermic reaction within the fuel cell. One example of an endothermicreaction includes steam reforming of a reformable fuel within a fuelcell anode and/or in an associated reforming stage, such as anintegrated reforming stage in a fuel cell stack. By providing additionalreformable fuel to the anode of the fuel cell (or to anintegrated/associated reforming stage), additional reforming can beperformed so that additional waste heat can be consumed. This can reducethe amount of temperature differential across the fuel cell, thusallowing the fuel cell to operate under an operating condition with anincreased amount of waste heat. The loss of electrical efficiency can beoffset by the creation of an additional product stream, such as syngasand/or H₂, that can be used for various purposes including additionalelectricity generation further expanding the power range of the system.

In various aspects, the amount of waste heat generated by a fuel cell,(V₀−V_(A))*I as defined above, can be at least about 30 mW/cm², such asat least about 40 mW/cm², or at least about 50 mW/cm², or at least about60 mW/cm², or at least about 70 mW/cm², or at least about 80 mW/cm², orat least about 100 mW/cm², or at least about 120 mW/cm², or at leastabout 140 mW/cm², or at least about 160 mW/cm², or at least about 180mW/cm². Additionally or alternately, the amount of waste heat generatedby a fuel cell can be less than about 250 mW/cm², such as less thanabout 200 mW/cm², or less than about 180 mW/cm², or less than about 165mW/cm², or less than about 150 mW/cm².

Although the amount of waste heat being generated can be relativelyhigh, such waste heat may not necessarily represent operating a fuelcell with poor efficiency. Instead, the waste heat can be generated dueto operating a fuel cell at an increased power density. Part ofimproving the power density of a fuel cell can include operating thefuel cell at a sufficiently high current density. In various aspects,the current density generated by the fuel cell can be at least about 150mA/cm², such as at least about 160 mA/cm², or at least about 170 mA/cm²,or at least about 180 mA/cm², or at least about 190 mA/cm², or at leastabout 200 mA/cm², or at least about 225 mA/cm², or at least about 250mA/cm². Additionally or alternately, the current density generated bythe fuel cell can be about 500 mA/cm² or less, such as 450 mA/cm², orless, or 400 mA/cm², or less or 350 mA/cm², or less or 300 mA/cm² orless.

In various aspects, to allow a fuel cell to be operated with increasedpower generation and increased generation of waste heat, an effectiveamount of an endothermic reaction (such as a reforming reaction) can beperformed. Alternatively, other endothermic reactions unrelated to anodeoperations can be used to utilize the waste heat by interspersing“plates” or stages into the fuel cell array that are in thermalcommunication but not fluid communication. The effective amount of theendothermic reaction can be performed in an associated reforming stage,an integrated reforming stage, an integrated stack element forperforming an endothermic reaction, or a combination thereof. Theeffective amount of the endothermic reaction can correspond to an amountsufficient to reduce the temperature rise from the fuel cell inlet tothe fuel cell outlet to about 100° C. or less, such as about 90° C. orless, or about 80° C. or less, or about 70° C. or less, or about 60° C.or less, or about 50° C. or less, or about 40° C. or less, or about 30°C. or less. Additionally or alternately, the effective amount of theendothermic reaction can correspond to an amount sufficient to cause atemperature decrease from the fuel cell inlet to the fuel cell outlet ofabout 100° C. or less, such as about 90° C. or less, or about 80° C. orless, or about 70° C. or less, or about 60° C. or less, or about 50° C.or less, or about 40° C. or less, or about 30° C. or less, or about 20°C. or less, or about 10° C. or less. A temperature decrease from thefuel cell inlet to the fuel cell outlet can occur when the effectiveamount of the endothermic reaction exceeds the waste heat generated.Additionally or alternately, this can correspond to having theendothermic reaction(s) (such as a combination of reforming and anotherendothermic reaction) consume at least about 40% of the waste heatgenerated by the fuel cell, such as consuming at least about 50% of thewaste heat, or at least about 60% of the waste heat, or at least about75% of the waste heat. Further additionally or alternately, theendothermic reaction(s) can consume about 95% of the waste heat or less,such as about 90% of the waste heat or less, or about 85% of the wasteheat or less.

As an addition, complement, and/or alternative to the fuel celloperating strategies described herein, a molten carbonate fuel cell(such as a fuel cell assembly) can be operated at conditionscorresponding to a decreased operating voltage and a low fuelutilization. In various aspects, the fuel cell can be operated at avoltage V_(A) of less than about 0.7 Volts, for example less than about0.68 V, less than about 0.67 V, less than about 0.66 V, or about 0.65 Vor less. Additionally or alternatively, the fuel cell can be operated ata voltage V_(A) of at least about 0.60, for example at least about 0.61,at least about 0.62, or at least about 0.63. In so doing, energy thatwould otherwise leave the fuel cell as electrical energy at high voltagecan remain within the cell as heat as the voltage is lowered. Thisadditional heat can allow for increased endothermic reactions to occur,for example increasing the CH₄ conversion to syngas.

DEFINITIONS

Syngas: In this description, syngas is defined as mixture of H₂ and COin any ratio. Optionally, H₂O and/or CO₂ may be present in the syngas.Optionally, inert compounds (such as nitrogen) and residual reformablefuel compounds may be present in the syngas. If components other than H₂and CO are present in the syngas, the combined volume percentage of H₂and CO in the syngas can be at least 25 vol % relative to the totalvolume of the syngas, such as at least 40 vol %, or at least 50 vol %,or at least 60 vol %. Additionally or alternately, the combined volumepercentage of H₂ and CO in the syngas can be 100 vol % or less, such as95 vol % or less or 90 vol % or less.

Reformable fuel: A reformable fuel is defined as a fuel that containscarbon-hydrogen bonds that can be reformed to generate H₂. Hydrocarbonsare examples of reformable fuels, as are other hydrocarbonaceouscompounds such as alcohols. Although CO and H₂O can participate in awater gas shift reaction to form hydrogen, CO is not considered areformable fuel under this definition.

Reformable hydrogen content: The reformable hydrogen content of a fuelis defined as the number of H₂ molecules that can be derived from a fuelby reforming the fuel and then driving the water gas shift reaction tocompletion to maximize H₂ production. It is noted that H₂ by definitionhas a reformable hydrogen content of 1, although H₂ itself is notdefined as a reformable fuel herein. Similarly, CO has a reformablehydrogen content of 1. Although CO is not strictly reformable, drivingthe water gas shift reaction to completion will result in exchange of aCO for an H₂. As examples of reformable hydrogen content for reformablefuels, the reformable hydrogen content of methane is 4H₂ molecules whilethe reformable hydrogen content of ethane is 7H₂ molecules. Moregenerally, if a fuel has the composition CxHyOz, then the reformablehydrogen content of the fuel at 100% reforming and water-gas shift isn(H₂ max reforming)=2x+y/2−z. Based on this definition, fuel utilizationwithin a cell can then be expressed as n(H₂ ox)/n(H₂ max reforming) Ofcourse, the reformable hydrogen content of a mixture of components canbe determined based on the reformable hydrogen content of the individualcomponents. The reformable hydrogen content of compounds that containother heteroatoms, such as oxygen, sulfur or nitrogen, can also becalculated in a similar manner.

Oxidation Reaction: In this discussion, the oxidation reaction withinthe anode of a fuel cell is defined as the reaction corresponding tooxidation of H₂ by reaction with CO₃ ²⁻ to form H₂O and CO₂. It is notedthat the reforming reaction within the anode, where a compoundcontaining a carbon-hydrogen bond is converted into H₂ and CO or CO₂, isexcluded from this definition of the oxidation reaction in the anode.The water-gas shift reaction is similarly outside of this definition ofthe oxidation reaction. It is further noted that references to acombustion reaction are defined as references to reactions where H₂ or acompound containing carbon-hydrogen bond(s) are reacted with O₂ to formH₂O and carbon oxides in a non-electrochemical burner, such as thecombustion zone of a combustion-powered generator.

Aspects of the invention can adjust anode fuel parameters to achieve adesired operating range for the fuel cell. Anode fuel parameters can becharacterized directly, and/or in relation to other fuel cell processesin the form of one or more ratios. For example, the anode fuelparameters can be controlled to achieve one or more ratios including afuel utilization, a fuel cell heating value utilization, a fuel surplusratio, a reformable fuel surplus ratio, a reformable hydrogen contentfuel ratio, and combinations thereof.

Fuel utilization: Fuel utilization is an option for characterizingoperation of the anode based on the amount of oxidized fuel relative tothe reformable hydrogen content of an input stream can be used to definea fuel utilization for a fuel cell. In this discussion, “fuelutilization” is defined as the ratio of the amount of hydrogen oxidizedin the anode for production of electricity (as described above) versusthe reformable hydrogen content of the anode input (including anyassociated reforming stages). Reformable hydrogen content has beendefined above as the number of H₂ molecules that can be derived from afuel by reforming the fuel and then driving the water gas shift reactionto completion to maximize H₂ production. For example, each methaneintroduced into an anode and exposed to steam reforming conditionsresults in generation of the equivalent of 4H₂ molecules at maxproduction. (Depending on the reforming and/or anode conditions, thereforming product can correspond to a non-water gas shifted product,where one or more of the H₂ molecules is present instead in the form ofa CO molecule.) Thus, methane is defined as having a reformable hydrogencontent of 4H₂ molecules. As another example, under this definitionethane has a reformable hydrogen content of 7H₂ molecules.

The utilization of fuel in the anode can also be characterized bydefining a heating value utilization based on a ratio of the LowerHeating Value of hydrogen oxidized in the anode due to the fuel cellanode reaction relative to the Lower Heating Value of all fuel deliveredto the anode and/or a reforming stage associated with the anode. The“fuel cell heating value utilization” as used herein can be computedusing the flow rates and Lower Heating Value (LHV) of the fuelcomponents entering and leaving the fuel cell anode. As such, fuel cellheating value utilization can be computed as(LHV(anode_in)−LHV(anode_out))/LHV(anode_in), where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In this definition, the LHV of a stream or flow may be computed as a sumof values for each fuel component in the input and/or output stream. Thecontribution of each fuel component to the sum can correspond to thefuel component's flow rate (e.g., mol/hr) multiplied by the fuelcomponent's LHV (e.g., joules/mol).

Lower Heating Value: The lower heating value is defined as the enthalpyof combustion of a fuel component to vapor phase, fully oxidizedproducts (i.e., vapor phase CO₂ and H₂O product). For example, any CO₂present in an anode input stream does not contribute to the fuel contentof the anode input, since CO₂ is already fully oxidized. For thisdefinition, the amount of oxidation occurring in the anode due to theanode fuel cell reaction is defined as oxidation of H₂ in the anode aspart of the electrochemical reaction in the anode, as defined above.

It is noted that, for the special case where the only fuel in the anodeinput flow is H₂, the only reaction involving a fuel component that cantake place in the anode represents the conversion of H₂ into H₂O. Inthis special case, the fuel utilization simplifies to (H₂-rate-in minusH₂-rate-out)/H₂-rate-in. In such a case, H₂ would be the only fuelcomponent, and so the H₂ LHV would cancel out of the equation. In themore general case, the anode feed may contain, for example, CH₄, H₂, andCO in various amounts. Because these species can typically be present indifferent amounts in the anode outlet, the summation as described abovecan be needed to determine the fuel utilization.

Alternatively or in addition to fuel utilization, the utilization forother reactants in the fuel cell can be characterized. For example, theoperation of a fuel cell can additionally or alternately becharacterized with regard to “CO₂ utilization” and/or “oxidant”utilization. The values for CO₂ utilization and/or oxidant utilizationcan be specified in a similar manner.

Fuel surplus ratio: Still another way to characterize the reactions in amolten carbonate fuel cell is by defining a utilization based on a ratioof the Lower Heating Value of all fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. This quantity will be referred to as a fuel surplus ratio. Assuch the fuel surplus ratio can be computed as (LHV(anode_in)/(LHV(anode_in)−LHV(anode_out)) where LHV(anode_in) andLHV(anode_out) refer to the LHV of the fuel components (such as H₂, CH₄,and/or CO) in the anode inlet and outlet streams or flows, respectively.In various aspects of the invention, a molten carbonate fuel cell can beoperated to have a fuel surplus ratio of at least about 1.0, such as atleast about 1.5, or at least about 2.0, or at least about 2.5, or atleast about 3.0, or at least about 4.0. Additionally or alternately, thefuel surplus ratio can be about 25.0 or less.

It is noted that not all of the reformable fuel in the input stream forthe anode may be reformed. Preferably, at least about 90% of thereformable fuel in the input stream to the anode (and/or into anassociated reforming stage) can be reformed prior to exiting the anode,such as at least about 95% or at least about 98%. In some alternativeaspects, the amount of reformable fuel that is reformed can be fromabout 75% to about 90%, such as at least about 80%.

The above definition for fuel surplus ratio provides a method forcharacterizing the amount of reforming occurring within the anode and/orreforming stage(s) associated with a fuel cell relative to the amount offuel consumed in the fuel cell anode for generation of electric power.

Optionally, the fuel surplus ratio can be modified to account forsituations where fuel is recycled from the anode output to the anodeinput. When fuel (such as H₂, CO, and/or unreformed or partiallyreformed hydrocarbons) is recycled from anode output to anode input,such recycled fuel components do not represent a surplus amount ofreformable or reformed fuel that can be used for other purposes.Instead, such recycled fuel components merely indicate a desire toreduce fuel utilization in a fuel cell.

Reformable fuel surplus ratio: Calculating a reformable fuel surplusratio is one option to account for such recycled fuel components is tonarrow the definition of surplus fuel, so that only the LHV ofreformable fuels is included in the input stream to the anode. As usedherein the “reformable fuel surplus ratio” is defined as the LowerHeating Value of reformable fuel delivered to the anode and/or areforming stage associated with the anode relative to the Lower HeatingValue of hydrogen oxidized in the anode due to the fuel cell anodereaction. Under the definition for reformable fuel surplus ratio, theLHV of any H₂ or CO in the anode input is excluded. Such an LHV ofreformable fuel can still be measured by characterizing the actualcomposition entering a fuel cell anode, so no distinction betweenrecycled components and fresh components needs to be made. Although somenon-reformed or partially reformed fuel may also be recycled, in mostaspects the majority of the fuel recycled to the anode can correspond toreformed products such as H₂ or CO. Expressed mathematically, thereformable fuel surplus ratio (R_(RFS))=LHV_(RF)/LHV_(OH), whereLHV_(RF) is the Lower Heating Value (LHV) of the reformable fuel andLHV_(OH) is the Lower Heating Value (LHV) of the hydrogen oxidized inthe anode. The LHV of the hydrogen oxidized in the anode may becalculated by subtracting the LHV of the anode outlet stream from theLHV of the anode inlet stream (e.g., LHV(anode_in)−LHV(anode_out)). Invarious aspects of the invention, a molten carbonate fuel cell can beoperated to have a reformable fuel surplus ratio of at least about 0.25,such as at least about 0.5, or at least about 1.0, or at least about1.5, or at least about 2.0, or at least about 2.5, or at least about3.0, or at least about 4.0. Additionally or alternately, the reformablefuel surplus ratio can be about 25.0 or less. It is noted that thisnarrower definition based on the amount of reformable fuel delivered tothe anode relative to the amount of oxidation in the anode candistinguish between two types of fuel cell operation methods that havelow fuel utilization. Some fuel cells achieve low fuel utilization byrecycling a substantial portion of the anode output back to the anodeinput. This recycle can allow any hydrogen in the anode input to be usedagain as an input to the anode. This can reduce the amount of reforming,as even though the fuel utilization is low for a single pass through thefuel cell, at least a portion of the unused fuel is recycled for use ina later pass. Thus, fuel cells with a wide variety of fuel utilizationvalues may have the same ratio of reformable fuel delivered to the anodereforming stage(s) versus hydrogen oxidized in the anode reaction. Inorder to change the ratio of reformable fuel delivered to the anodereforming stages relative to the amount of oxidation in the anode,either an anode feed with a native content of non-reformable fuel needsto be identified, or unused fuel in the anode output needs to bewithdrawn for other uses, or both.

Reformable hydrogen surplus ratio: Still another option forcharacterizing the operation of a fuel cell is based on a “reformablehydrogen surplus ratio.” The reformable fuel surplus ratio defined aboveis defined based on the lower heating value of reformable fuelcomponents. The reformable hydrogen surplus ratio is defined as thereformable hydrogen content of reformable fuel delivered to the anodeand/or a reforming stage associated with the anode relative to thehydrogen reacted in the anode due to the fuel cell anode reaction. Assuch, the “reformable hydrogen surplus ratio” can be computed as(RFC(reformable_anode_in)/(RFC(reformable_anode_in)−RFC(anode_out)),where RFC(reformable_anode_in) refers to the reformable hydrogen contentof reformable fuels in the anode inlet streams or flows, while RFC(anode_out) refers to the reformable hydrogen content of the fuelcomponents (such as H₂, CH₄, and/or CO) in the anode inlet and outletstreams or flows. The RFC can be expressed in moles/s, moles/hr, orsimilar. An example of a method for operating a fuel cell with a largeratio of reformable fuel delivered to the anode reforming stage(s)versus amount of oxidation in the anode can be a method where excessreforming is performed in order to balance the generation andconsumption of heat in the fuel cell. Reforming a reformable fuel toform H₂ and CO is an endothermic process. This endothermic reaction canbe countered by the generation of electrical current in the fuel cell,which can also produce excess heat corresponding (roughly) to thedifference between the amount of heat generated by the anode oxidationreaction and the carbonate formation reaction and the energy that exitsthe fuel cell in the form of electric current. The excess heat per moleof hydrogen involved in the anode oxidation reaction/carbonate formationreaction can be greater than the heat absorbed to generate a mole ofhydrogen by reforming. As a result, a fuel cell operated underconventional conditions can exhibit a temperature increase from inlet tooutlet. Instead of this type of conventional operation, the amount offuel reformed in the reforming stages associated with the anode can beincreased. For example, additional fuel can be reformed so that the heatgenerated by the exothermic fuel cell reactions can be (roughly)balanced by the heat consumed in reforming, or even the heat consumed byreforming can be beyond the excess heat generated by the fuel oxidation,resulting in a temperature drop across the fuel cell. This can result ina substantial excess of hydrogen relative to the amount needed forelectrical power generation. As one example, a feed to the anode inletof a fuel cell or an associated reforming stage can be substantiallycomposed of reformable fuel, such as a substantially pure methane feed.During conventional operation for electric power generation using such afuel, a molten carbonate fuel cell can be operated with a fuelutilization of about 75%. This means that about 75% (or ¾) of the fuelcontent delivered to the anode is used to form hydrogen that is thenreacted in the anode with carbonate ions to form H₂O and CO₂. Inconventional operation, the remaining about 25% of the fuel content canbe reformed to H₂ within the fuel cell (or can pass through the fuelcell unreacted for any CO or H₂ in the fuel), and then combusted outsideof the fuel cell to form H₂O and CO₂ to provide heat for the cathodeinlet to the fuel cell. The reformable hydrogen surplus ratio in thissituation can be 4/(4−1)=4/3.

Electrical efficiency: As used herein, the term “electrical efficiency”(“EE”) is defined as the electrochemical power produced by the fuel celldivided by the rate of Lower Heating Value (“LHV”) of fuel input to thefuel cell. The fuel inputs to the fuel cell includes both fuel deliveredto the anode as well as any fuel used to maintain the temperature of thefuel cell, such as fuel delivered to a burner associated with a fuelcell. In this description, the power produced by the fuel may bedescribed in terms of LHV(el) fuel rate.

Electrochemical power: As used herein, the term “electrochemical power”or LHV(el) is the power generated by the circuit connecting the cathodeto the anode in the fuel cell and the transfer of carbonate ions acrossthe fuel cell's electrolyte. Electrochemical power excludes powerproduced or consumed by equipment upstream or downstream from the fuelcell. For example, electricity produced from heat in a fuel cell exhauststream is not considered part of the electrochemical power. Similarly,power generated by a gas turbine or other equipment upstream of the fuelcell is not part of the electrochemical power generated. The“electrochemical power” does not take electrical power consumed duringoperation of the fuel cell into account, or any loss incurred byconversion of the direct current to alternating current. In other words,electrical power used to supply the fuel cell operation or otherwiseoperate the fuel cell is not subtracted from the direct current powerproduced by the fuel cell. As used herein, the power density is thecurrent density multiplied by voltage. As used herein, the total fuelcell power is the power density multiplied by the fuel cell area.

Fuel inputs: As used herein, the term “anode fuel input,” designated asLHV(anode_in), is the amount of fuel within the anode inlet stream. Theterm “fuel input”, designated as LHV(in), is the total amount of fueldelivered to the fuel cell, including both the amount of fuel within theanode inlet stream and the amount of fuel used to maintain thetemperature of the fuel cell. The fuel may include both reformable andnonreformable fuels, based on the definition of a reformable fuelprovided herein. Fuel input is not the same as fuel utilization.

Total fuel cell efficiency: As used herein, the term “total fuel cellefficiency” (“TFCE”) is defined as: the electrochemical power generatedby the fuel cell, plus the rate of LHV of syngas produced by the fuelcell, divided by the rate of LHV of fuel input to the anode. In otherwords, TFCE=(LHV(el)+LHV(sg net))/LHV(anode_in), where LHV(anode_in)refers to rate at which the LHV of the fuel components (such as H₂, CH₄,and/or CO) delivered to the anode and LHV(sg net) refers to a rate atwhich syngas (H₂, CO) is produced in the anode, which is the differencebetween syngas input to the anode and syngas output from the anode.LHV(el) describes the electrochemical power generation of the fuel cell.The total fuel cell efficiency excludes heat generated by the fuel cellthat is put to beneficial use outside of the fuel cell. In operation,heat generated by the fuel cell may be put to beneficial use bydownstream equipment. For example, the heat may be used to generateadditional electricity or to heat water. These uses, when they occurapart from the fuel cell, are not part of the total fuel cellefficiency, as the term is used in this application. The total fuel cellefficiency is for the fuel cell operation only, and does not includepower production, or consumption, upstream, or downstream, of the fuelcell.

Chemical efficiency: As used herein, the term “chemical efficiency”, isdefined as the lower heating value of H₂ and CO in the anode exhaust ofthe fuel cell, or LHV(sg out), divided by the fuel input, or LHV(in).

Neither the electrical efficiency nor the total system efficiency takesthe efficiency of upstream or downstream processes into consideration.For example, it may be advantageous to use turbine exhaust as a sourceof CO₂ for the fuel cell cathode. In this arrangement, the efficiency ofthe turbine is not considered as part of the electrical efficiency orthe total fuel cell efficiency calculation. Similarly, outputs from thefuel cell may be recycled as inputs to the fuel cell. A recycle loop isnot considered when calculating electrical efficiency or the total fuelcell efficiency in single pass mode.

Syngas produced: As used herein, the term “syngas produced” is thedifference between syngas input to the anode and syngas output from theanode. Syngas may be used as an input, or fuel, for the anode, at leastin part. For example, a system may include an anode recycle loop thatreturns syngas from the anode exhaust to the anode inlet where it issupplemented with natural gas or other suitable fuel. Syngas producedLHV (sg net)=(LHV(sg out)−LHV(sg in)), where LHV(sg in) and LHV(sg out)refer to the LHV of the syngas in the anode inlet and syngas in theanode outlet streams or flows, respectively. It is noted that at least aportion of the syngas produced by the reforming reactions within ananode can typically be utilized in the anode to produce electricity. Thehydrogen utilized to produce electricity is not included in thedefinition of “syngas produced” because it does not exit the anode. Asused herein, the term “syngas ratio” is the LHV of the net syngasproduced divided by the LHV of the fuel input to the anode or LHV (sgnet)/LHV(anode in). Molar flow rates of syngas and fuel can be usedinstead of LHV to express a molar-based syngas ratio and a molar-basedsyngas produced.

Steam to carbon ratio (S/C): As used herein, the steam to carbon ratio(S/C) is the molar ratio of steam in a flow to reformable carbon in theflow. Carbon in the form of CO and CO₂ are not included as reformablecarbon in this definition. The steam to carbon ratio can be measuredand/or controlled at different points in the system. For example, thecomposition of an anode inlet stream can be manipulated to achieve a S/Cthat is suitable for reforming in the anode. The S/C can be given as themolar flow rate of H₂O divided by the product of the molar flow rate offuel multiplied by the number of carbon atoms in the fuel, e.g. one formethane. Thus, S/C=f_(H20)/(f_(CH4)×#C), where f_(H20) is the molar flowrate of water, where f_(CH4) is the molar flow rate of methane (or otherfuel) and #C is the number of carbons in the fuel.

EGR ratio: Aspects of the invention can use a turbine in partnershipwith a fuel cell. The combined fuel cell and turbine system may includeexhaust gas recycle (“EGR”). In an EGR system, at least a portion of theexhaust gas generated by the turbine can be sent to a heat recoverygenerator. Another portion of the exhaust gas can be sent to the fuelcell. The EGR ratio describes the amount of exhaust gas routed to thefuel cell versus the total exhaust gas routed to either the fuel cell orheat recovery generator. As used herein, the “EGR ratio” is the flowrate for the fuel cell bound portion of the exhaust gas divided by thecombined flow rate for the fuel cell bound portion and the recoverybound portion, which is sent to the heat recovery generator.

In various aspects of the invention, a molten carbonate fuel cell (MCFC)can be used to facilitate separation of CO₂ from a CO₂-containing streamwhile also generating additional electrical power. The CO₂ separationcan be further enhanced by taking advantage of synergies with thecombustion-based power generator that can provide at least a portion ofthe input feed to the cathode portion of the fuel cell.

Fuel Cell and Fuel Cell Components: In this discussion, a fuel cell cancorrespond to a single cell, with an anode and a cathode separated by anelectrolyte. The anode and cathode can receive input gas flows tofacilitate the respective anode and cathode reactions for transportingcharge across the electrolyte and generating electricity. A fuel cellstack can represent a plurality of cells in an integrated unit. Althougha fuel cell stack can include multiple fuel cells, the fuel cells cantypically be connected in parallel and can function (approximately) asif they collectively represented a single fuel cell of a larger size.When an input flow is delivered to the anode or cathode of a fuel cellstack, the fuel stack can include flow channels for dividing the inputflow between each of the cells in the stack and flow channels forcombining the output flows from the individual cells. In thisdiscussion, a fuel cell array can be used to refer to a plurality offuel cells (such as a plurality of fuel cell stacks) that are arrangedin series, in parallel, or in any other convenient manner (e.g., in acombination of series and parallel). A fuel cell array can include oneor more stages of fuel cells and/or fuel cell stacks, where theanode/cathode output from a first stage may serve as the anode/cathodeinput for a second stage. It is noted that the anodes in a fuel cellarray do not have to be connected in the same way as the cathodes in thearray. For convenience, the input to the first anode stage of a fuelcell array may be referred to as the anode input for the array, and theinput to the first cathode stage of the fuel cell array may be referredto as the cathode input to the array. Similarly, the output from thefinal anode/cathode stage may be referred to as the anode/cathode outputfrom the array.

It should be understood that reference to use of a fuel cell hereintypically denotes a “fuel cell stack” composed of individual fuel cells,and more generally refers to use of one or more fuel cell stacks influid communication. Individual fuel cell elements (plates) cantypically be “stacked” together in a rectangular array called a “fuelcell stack”. This fuel cell stack can typically take a feed stream anddistribute reactants among all of the individual fuel cell elements andcan then collect the products from each of these elements. When viewedas a unit, the fuel cell stack in operation can be taken as a whole eventhough composed of many (often tens or hundreds) of individual fuel cellelements. These individual fuel cell elements can typically have similarvoltages (as the reactant and product concentrations are similar), andthe total power output can result from the summation of all of theelectrical currents in all of the cell elements, when the elements areelectrically connected in series. Stacks can also be arranged in aseries arrangement to produce high voltages. A parallel arrangement canboost the current. If a sufficiently large volume fuel cell stack isavailable to process a given exhaust flow, the systems and methodsdescribed herein can be used with a single molten carbonate fuel cellstack. In other aspects of the invention, a plurality of fuel cellstacks may be desirable or needed for a variety of reasons.

For the purposes of this invention, unless otherwise specified, the term“fuel cell” should be understood to also refer to and/or is defined asincluding a reference to a fuel cell stack composed of set of one ormore individual fuel cell elements for which there is a single input andoutput, as that is the manner in which fuel cells are typically employedin practice. Similarly, the term fuel cells (plural), unless otherwisespecified, should be understood to also refer to and/or is defined asincluding a plurality of separate fuel cell stacks. In other words, allreferences within this document, unless specifically noted, can referinterchangeably to the operation of a fuel cell stack as a “fuel cell”.For example, the volume of exhaust generated by a commercial scalecombustion generator may be too large for processing by a fuel cell(i.e., a single stack) of conventional size. In order to process thefull exhaust, a plurality of fuel cells (i.e., two or more separate fuelcells or fuel cell stacks) can be arranged in parallel, so that eachfuel cell can process (roughly) an equal portion of the combustionexhaust. Although multiple fuel cells can be used, each fuel cell cantypically be operated in a generally similar manner, given its (roughly)equal portion of the combustion exhaust.

“Internal reforming” and “external reforming”: A fuel cell or fuel cellstack may include one or more internal reforming sections. As usedherein, the term “internal reforming” refers to fuel reforming occurringwithin the body of a fuel cell, a fuel cell stack, or otherwise within afuel cell assembly. External reforming, which is often used inconjunction with a fuel cell, occurs in a separate piece of equipmentthat is located outside of the fuel cell stack. In other words, the bodyof the external reformer is not in direct physical contact with the bodyof a fuel cell or fuel cell stack. In a typical set up, the output fromthe external reformer can be fed to the anode inlet of a fuel cell.Unless otherwise noted specifically, the reforming described within thisapplication is internal reforming.

Internal reforming may occur within a fuel cell anode. Internalreforming can additionally or alternately occur within an internalreforming element integrated within a fuel cell assembly. The integratedreforming element may be located between fuel cell elements within afuel cell stack. In other words, one of the trays in the stack can be areforming section instead of a fuel cell element. In one aspect, theflow arrangement within a fuel cell stack directs fuel to the internalreforming elements and then into the anode portion of the fuel cells.Thus, from a flow perspective, the internal reforming elements and fuelcell elements can be arranged in series within the fuel cell stack. Asused herein, the term “anode reforming” is fuel reforming that occurswithin an anode. As used herein, the term “internal reforming” isreforming that occurs within an integrated reforming element and not inan anode section.

In some aspects, a reforming stage that is internal to a fuel cellassembly can be considered to be associated with the anode(s) in thefuel cell assembly. In some alternative aspects, for a reforming stagein a fuel cell stack that can be associated with an anode (such asassociated with multiple anodes), a flow path can be available so thatthe output flow from the reforming stage is passed into at least oneanode. This can correspond to having an initial section of a fuel cellplate not in contact with the electrolyte and instead can serve just asa reforming catalyst. Another option for an associated reforming stagecan be to have a separate integrated reforming stage as one of theelements in a fuel cell stack, where the output from the integratedreforming stage can be returned to the input side of one or more of thefuel cells in the fuel cell stack.

From a heat integration standpoint, a characteristic height in a fuelcell stack can be the height of an individual fuel cell stack element.It is noted that the separate reforming stage and/or a separateendothermic reaction stage could have a different height in the stackthan a fuel cell. In such a scenario, the height of a fuel cell elementcan be used as the characteristic height. In some aspects, an integratedendothermic reaction stage can be defined as a stage that is heatintegrated with one or more fuel cells, so that the integratedendothermic reaction stage can use the heat from the fuel cells as aheat source for the endothermic reaction. Such an integrated endothermicreaction stage can be defined as being positioned less than 5 times theheight of a stack element from any fuel cells providing heat to theintegrated stage. For example, an integrated endothermic reaction stage(such as a reforming stage) can be positioned less than 5 times theheight of a stack element from any fuel cells that are heat integrated,such as less than 3 times the height of a stack element. In thisdiscussion, an integrated reforming stage and/or integrated endothermicreaction stage that represent an adjacent stack element to a fuel cellelement can be defined as being about one stack element height or lessaway from the adjacent fuel cell element.

In some aspects, a separate reforming stage that is heat integrated witha fuel cell element can correspond to a reforming stage associated withthe fuel cell element. In such aspects, an integrated fuel cell elementcan provide at least a portion of the heat to the associated reformingstage, and the associated reforming stage can provide at least a portionof the reforming stage output to the integrated fuel cell as a fuelstream. In other aspects, a separate reforming stage can be integratedwith a fuel cell for heat transfer without being associated with thefuel cell. In this type of situation, the separate reforming stage canreceive heat from the fuel cell, but the decision can be made not to usethe output of the reforming stage as an input to the fuel cell. Instead,the decision can be made to use the output of such a reforming stage foranother purpose, such as directly adding the output to the anode exhauststream, and/or for forming a separate output stream from the fuel cellassembly.

More generally, a separate stack element in a fuel cell stack can beused to perform any convenient type of endothermic reaction that cantake advantage of the waste heat provided by integrated fuel cell stackelements. Instead of plates suitable for performing a reforming reactionon a hydrocarbon fuel stream, a separate stack element can have platessuitable for catalyzing another type of endothermic reaction. A manifoldor other arrangement of inlet conduits in the fuel cell stack can beused to provide an appropriate input flow to each stack element. Asimilar manifold or other arrangement of outlet conduits canadditionally or alternately be used to withdraw the output flows fromeach stack element. Optionally, the output flows from a endothermicreaction stage in a stack can be withdrawn from the fuel cell stackwithout having the output flow pass through a fuel cell anode. In suchan optional aspect, the products of the exothermic reaction cantherefore exit from the fuel cell stack without passing through a fuelcell anode. Examples of other types of endothermic reactions that can beperformed in stack elements in a fuel cell stack can include, withoutlimitation, ethanol dehydration to form ethylene and ethane cracking.

Recycle: As defined herein, recycle of a portion of a fuel cell output(such as an anode exhaust or a stream separated or withdrawn from ananode exhaust) to a fuel cell inlet can correspond to a direct orindirect recycle stream. A direct recycle of a stream to a fuel cellinlet is defined as recycle of the stream without passing through anintermediate process, while an indirect recycle involves recycle afterpassing a stream through one or more intermediate processes. Forexample, if the anode exhaust is passed through a CO₂ separation stageprior to recycle, this is considered an indirect recycle of the anodeexhaust. If a portion of the anode exhaust, such as an H₂ streamwithdrawn from the anode exhaust, is passed into a gasifier forconverting coal into a fuel suitable for introduction into the fuelcell, then that is also considered an indirect recycle.

Anode Inputs and Outputs

In various aspects of the invention, the MCFC array can be fed by a fuelreceived at the anode inlet that comprises, for example, both hydrogenand a hydrocarbon such as methane (or alternatively a hydrocarbonaceousor hydrocarbon-like compound that may contain heteroatoms different fromC and H). Most of the methane (or other hydrocarbonaceous orhydrocarbon-like compound) fed to the anode can typically be freshmethane. In this description, a fresh fuel such as fresh methane refersto a fuel that is not recycled from another fuel cell process. Forexample, methane recycled from the anode outlet stream back to the anodeinlet may not be considered “fresh” methane, and can instead bedescribed as reclaimed methane. The fuel source used can be shared withother components, such as a turbine that uses a portion of the fuelsource to provide a CO₂-containing stream for the cathode input. Thefuel source input can include water in a proportion to the fuelappropriate for reforming the hydrocarbon (or hydrocarbon-like) compoundin the reforming section that generates hydrogen. For example, ifmethane is the fuel input for reforming to generate H₂, the molar ratioof water to fuel can be from about one to one to about ten to one, suchas at least about two to one. A ratio of four to one or greater istypical for external reforming, but lower values can be typical forinternal reforming To the degree that H₂ is a portion of the fuelsource, in some optional aspects no additional water may be needed inthe fuel, as the oxidation of H₂ at the anode can tend to produce H₂Othat can be used for reforming the fuel. The fuel source can alsooptionally contain components incidental to the fuel source (e.g., anatural gas feed can contain some content of CO₂ as an additionalcomponent). For example, a natural gas feed can contain CO₂, N₂, and/orother inert (noble) gases as additional components. Optionally, in someaspects the fuel source may also contain CO, such as CO from a recycledportion of the anode exhaust. An additional or alternate potentialsource for CO in the fuel into a fuel cell assembly can be CO generatedby steam reforming of a hydrocarbon fuel performed on the fuel prior toentering the fuel cell assembly.

More generally, a variety of types of fuel streams may be suitable foruse as an input stream for the anode of a molten carbonate fuel cell.Some fuel streams can correspond to streams containing hydrocarbonsand/or hydrocarbon-like compounds that may also include heteroatomsdifferent from C and H. In this discussion, unless otherwise specified,a reference to a fuel stream containing hydrocarbons for an MCFC anodeis defined to include fuel streams containing such hydrocarbon-likecompounds. Examples of hydrocarbon (including hydrocarbon-like) fuelstreams include natural gas, streams containing C1-C4 carbon compounds(such as methane or ethane), and streams containing heavier C5+hydrocarbons (including hydrocarbon-like compounds), as well ascombinations thereof. Still other additional or alternate examples ofpotential fuel streams for use in an anode input can include biogas-typestreams, such as methane produced from natural (biological)decomposition of organic material.

In some aspects, a molten carbonate fuel cell can be used to process aninput fuel stream, such as a natural gas and/or hydrocarbon stream, witha low energy content due to the presence of diluent compounds. Forexample, some sources of methane and/or natural gas are sources that caninclude substantial amounts of either CO₂ or other inert molecules, suchas nitrogen, argon, or helium. Due to the presence of elevated amountsof CO₂ and/or inerts, the energy content of a fuel stream based on thesource can be reduced. Using a low energy content fuel for a combustionreaction (such as for powering a combustion-powered turbine) can posedifficulties. However, a molten carbonate fuel cell can generate powerbased on a low energy content fuel source with a reduced or minimalimpact on the efficiency of the fuel cell. The presence of additionalgas volume can require additional heat for raising the temperature ofthe fuel to the temperature for reforming and/or the anode reaction.Additionally, due to the equilibrium nature of the water gas shiftreaction within a fuel cell anode, the presence of additional CO₂ canhave an impact on the relative amounts of H₂ and CO present in the anodeoutput. However, the inert compounds otherwise can have only a minimaldirect impact on the reforming and anode reactions. The amount of CO₂and/or inert compounds in a fuel stream for a molten carbonate fuelcell, when present, can be at least about 1 vol %, such as at leastabout 2 vol %, or at least about 5 vol %, or at least about 10 vol %, orat least about 15 vol %, or at least about 20 vol %, or at least about25 vol %, or at least about 30 vol %, or at least about 35 vol %, or atleast about 40 vol %, or at least about 45 vol %, or at least about 50vol %, or at least about 75 vol %. Additionally or alternately, theamount of CO₂ and/or inert compounds in a fuel stream for a moltencarbonate fuel cell can be about 90 vol % or less, such as about 75 vol% or less, or about 60 vol % or less, or about 50 vol % or less, orabout 40 vol % or less, or about 35 vol % or less.

Yet other examples of potential sources for an anode input stream cancorrespond to refinery and/or other industrial process output streams.For example, coking is a common process in many refineries forconverting heavier compounds to lower boiling ranges. Coking typicallyproduces an off-gas containing a variety of compounds that are gases atroom temperature, including CO and various C1-C4 hydrocarbons. Thisoff-gas can be used as at least a portion of an anode input stream.Other refinery off-gas streams can additionally or alternately besuitable for inclusion in an anode input stream, such as light ends(C1-C4) generated during cracking or other refinery processes. Stillother suitable refinery streams can additionally or alternately includerefinery streams containing CO or CO₂ that also contain H₂ and/orreformable fuel compounds.

Still other potential sources for an anode input can additionally oralternately include streams with increased water content. For example,an ethanol output stream from an ethanol plant (or another type offermentation process) can include a substantial portion of H₂O prior tofinal distillation. Such H₂O can typically cause only minimal impact onthe operation of a fuel cell. Thus, a fermentation mixture of alcohol(or other fermentation product) and water can be used as at least aportion of an anode input stream.

Biogas, or digester gas, is another additional or alternate potentialsource for an anode input. Biogas may primarily comprise methane and CO₂and is typically produced by the breakdown or digestion of organicmatter. Anaerobic bacteria may be used to digest the organic matter andproduce the biogas. Impurities, such as sulfur-containing compounds, maybe removed from the biogas prior to use as an anode input.

The output stream from an MCFC anode can include H₂O, CO₂, CO, and H₂.Optionally, the anode output stream could also have unreacted fuel (suchas H₂ or CH₄) or inert compounds in the feed as additional outputcomponents. Instead of using this output stream as a fuel source toprovide heat for a reforming reaction or as a combustion fuel forheating the cell, one or more separations can be performed on the anodeoutput stream to separate the CO₂ from the components with potentialvalue as inputs to another process, such as H₂ or CO. The H₂ and/or COcan be used as a syngas for chemical synthesis, as a source of hydrogenfor chemical reaction, and/or as a fuel with reduced greenhouse gasemissions.

In various aspects, the composition of the output stream from the anodecan be impacted by several factors. Factors that can influence the anodeoutput composition can include the composition of the input stream tothe anode, the amount of current generated by the fuel cell, and/or thetemperature at the exit of the anode. The temperature of at the anodeexit can be relevant due to the equilibrium nature of the water gasshift reaction. In a typical anode, at least one of the plates formingthe wall of the anode can be suitable for catalyzing the water gas shiftreaction. As a result, if a) the composition of the anode input streamis known, b) the extent of reforming of reformable fuel in the anodeinput stream is known, and c) the amount of carbonate transported fromthe cathode to anode (corresponding to the amount of electrical currentgenerated) is known, the composition of the anode output can bedetermined based on the equilibrium constant for the water gas shiftreaction.

K_(eq)=[CO₂][H₂]/[CO][H₂O]

In the above equation, K_(eq) is the equilibrium constant for thereaction at a given temperature and pressure, and [X] is the partialpressure of component X. Based on the water gas shift reaction, it canbe noted that an increased CO₂ concentration in the anode input can tendto result in additional CO formation (at the expense of H₂) while anincreased H₂O concentration can tend to result in additional H₂formation (at the expense of CO).

To determine the composition at the anode output, the composition of theanode input can be used as a starting point. This composition can thenbe modified to reflect the extent of reforming of any reformable fuelsthat can occur within the anode. Such reforming can reduce thehydrocarbon content of the anode input in exchange for increasedhydrogen and CO₂. Next, based on the amount of electrical currentgenerated, the amount of H₂ in the anode input can be reduced inexchange for additional H₂O and CO₂. This composition can then beadjusted based on the equilibrium constant for the water gas shiftreaction to determine the exit concentrations for H₂, CO, CO₂, and H₂O.

Table 1 shows the anode exhaust composition at different fuelutilizations for a typical type of fuel. The anode exhaust compositioncan reflect the combined result of the anode reforming reaction, watergas shift reaction, and the anode oxidation reaction. The outputcomposition values in Table 1 were calculated by assuming an anode inputcomposition with an about 2 to 1 ratio of steam (H₂O) to carbon(reformable fuel). The reformable fuel was assumed to be methane, whichwas assumed to be 100% reformed to hydrogen. The initial CO₂ and H₂concentrations in the anode input were assumed to be negligible, whilethe input N₂ concentration was about 0.5%. The fuel utilization U_(f)(as defined herein) was allowed to vary from about 35% to about 70% asshown in the table. The exit temperature for the fuel cell anode wasassumed to be about 650° C. for purposes of determining the correctvalue for the equilibrium constant.

TABLE 1 Anode Exhaust Composition Uf % 35% 40% 45% 50% 55% 60% 65% 70%Anode Exhaust Composition H₂O %, wet 32.5% 34.1% 35.5% 36.7% 37.8% 38.9%39.8% 40.5% CO₂ %, wet 26.7% 29.4% 32.0% 34.5% 36.9% 39.3% 41.5% 43.8%H₂ %, wet 29.4% 26.0% 22.9% 20.0% 17.3% 14.8% 12.5% 10.4% CO %, wet10.8% 10.0% 9.2% 8.4% 7.5% 6.7% 5.8% 4.9% N₂ %, wet 0.5% 0.5% 0.5% 0.4%0.4% 0.4% 0.4% 0.4% CO₂ %, dry 39.6% 44.6% 49.6% 54.5% 59.4% 64.2% 69.0%73.7% H₂ %, dry 43.6% 39.4% 35.4% 31.5% 27.8% 24.2% 20.7% 17.5% CO %,dry 16.1% 15.2% 14.3% 13.2% 12.1% 10.9% 9.7% 8.2% N₂ %, dry 0.7% 0.7%0.7% 0.7% 0.7% 0.7% 0.7% 0.7% H₂/CO 2.7 2.6 2.5 2.4 2.3 2.2 2.1 2.1 (H₂− CO₂)/ 0.07 −0.09 −0.22 −0.34 −0.44 −0.53 −0.61 −0.69 (CO + CO₂)

Table 1 shows anode output compositions for a particular set ofconditions and anode input composition. More generally, in variousaspects the anode output can include about 10 vol % to about 50 vol %H₂O. The amount of H₂O can vary greatly, as H₂O in the anode can beproduced by the anode oxidation reaction. If an excess of H₂O beyondwhat is needed for reforming is introduced into the anode, the excessH₂O can typically pass through largely unreacted, with the exception ofH₂O consumed (or generated) due to fuel reforming and the water gasshift reaction. The CO₂ concentration in the anode output can also varywidely, such as from about 20 vol % to about 50 vol % CO₂. The amount ofCO₂ can be influenced by both the amount of electrical current generatedas well as the amount of CO₂ in the anode input flow. The amount of H₂in the anode output can additionally or alternately be from about 10 vol% H₂ to about 50 vol % H₂, depending on the fuel utilization in theanode. At the anode output, the amount of CO can be from about 5 vol %to about 20 vol %. It is noted that the amount of CO relative to theamount of H₂ in the anode output for a given fuel cell can be determinedin part by the equilibrium constant for the water gas shift reaction atthe temperature and pressure present in the fuel cell. The anode outputcan further additionally or alternately include 5 vol % or less ofvarious other components, such as N₂, CH₄ (or other unreactedcarbon-containing fuels), and/or other components.

Optionally, one or more water gas shift reaction stages can be includedafter the anode output to convert CO and H₂O in the anode output intoCO₂ and H₂, if desired. The amount of H₂ present in the anode output canbe increased, for example, by using a water gas shift reactor at lowertemperature to convert H₂O and CO present in the anode output into H₂and CO₂. Alternatively, the temperature can be raised and the water-gasshift reaction can be reversed, producing more CO and H₂O from H₂ andCO₂. Water is an expected output of the reaction occurring at the anode,so the anode output can typically have an excess of H₂O relative to theamount of CO present in the anode output. Alternatively, H₂O can beadded to the stream after the anode exit but before the water gas shiftreaction. CO can be present in the anode output due to incomplete carbonconversion during reforming and/or due to the equilibrium balancingreactions between H₂O, CO, H₂, and CO₂ (i.e., the water-gas shiftequilibrium) under either reforming conditions or the conditions presentduring the anode reaction. A water gas shift reactor can be operatedunder conditions to drive the equilibrium further in the direction offorming CO₂ and H₂ at the expense of CO and H₂O. Higher temperatures cantend to favor the formation of CO and H₂O. Thus, one option foroperating the water gas shift reactor can be to expose the anode outputstream to a suitable catalyst, such as a catalyst including iron oxide,zinc oxide, copper on zinc oxide, or the like, at a suitabletemperature, e.g., between about 190° C. to about 210° C. Optionally,the water-gas shift reactor can include two stages for reducing the COconcentration in an anode output stream, with a first higher temperaturestage operated at a temperature from at least about 300° C. to about375° C. and a second lower temperature stage operated at a temperatureof about 225° C. or less, such as from about 180° C. to about 210° C. Inaddition to increasing the amount of H₂ present in the anode output, thewater-gas shift reaction can additionally or alternately increase theamount of CO₂ at the expense of CO. This can exchangedifficult-to-remove carbon monoxide (CO) for carbon dioxide, which canbe more readily removed by condensation (e.g., cryogenic removal),chemical reaction (such as amine removal), and/or other CO₂ removalmethods. Additionally or alternately, it may be desirable to increasethe CO content present in the anode exhaust in order to achieve adesired ratio of H₂ to CO.

After passing through the optional water gas shift reaction stage, theanode output can be passed through one or more separation stages forremoval of water and/or CO₂ from the anode output stream. For example,one or more CO₂ output streams can be formed by performing CO₂separation on the anode output using one or more methods individually orin combination. Such methods can be used to generate CO₂ outputstream(s) having a CO₂ content of 90 vol % or greater, such as at least95% vol % CO₂, or at least 98 vol % CO₂. Such methods can recover aboutat least about 70% of the CO₂ content of the anode output, such as atleast about 80% of the CO₂ content of the anode output, or at leastabout 90%. Alternatively, in some aspects it may be desirable to recoveronly a portion of the CO₂ within an anode output stream, with therecovered portion of CO₂ being about 33% to about 90% of the CO₂ in theanode output, such as at least about 40%, or at least about 50%. Forexample, it may be desirable to retain some CO₂ in the anode output flowso that a desired composition can be achieved in a subsequent water gasshift stage. Suitable separation methods may comprise use of a physicalsolvent (e.g., Selexol™ or Rectisol™); amines or other bases (e.g., MEAor MDEA); refrigeration (e.g., cryogenic separation); pressure swingadsorption; vacuum swing adsorption; and combinations thereof. Acryogenic CO₂ separator can be an example of a suitable separator. Asthe anode output is cooled, the majority of the water in the anodeoutput can be separated out as a condensed (liquid) phase. Furthercooling and/or pressurizing of the water-depleted anode output flow canthen separate high purity CO₂, as the other remaining components in theanode output flow (such as H₂, N₂, CH₄) do not tend to readily formcondensed phases. A cryogenic CO₂ separator can recover between about33% and about 90% of the CO₂ present in a flow, depending on theoperating conditions.

Removal of water from the anode exhaust to form one or more water outputstreams can also be beneficial, whether prior to, during, or afterperforming CO₂ separation. The amount of water in the anode output canvary depending on operating conditions selected. For example, thesteam-to-carbon ratio established at the anode inlet can affect thewater content in the anode exhaust, with high steam-to-carbon ratiostypically resulting in a large amount of water that can pass through theanode unreacted and/or reacted only due to the water gas shiftequilibrium in the anode. Depending on the aspect, the water content inthe anode exhaust can correspond to up to about 30% or more of thevolume in the anode exhaust. Additionally or alternately, the watercontent can be about 80% or less of the volume of the anode exhaust.While such water can be removed by compression and/or cooling withresulting condensation, the removal of this water can require extracompressor power and/or heat exchange surface area and excessive coolingwater. One beneficial way to remove a portion of this excess water canbe based on use of an adsorbent bed that can capture the humidity fromthe moist anode effluent and can then be ‘regenerated’ using dry anodefeed gas, in order to provide additional water for the anode feed.HVAC-style (heating, ventilation, and air conditioning) adsorptionwheels design can be applicable, because anode exhaust and inlet can besimilar in pressure, and minor leakage from one stream to the other canhave minimal impact on the overall process. In embodiments where CO₂removal is performed using a cryogenic process, removal of water priorto or during CO₂ removal may be desirable, including removal bytriethyleneglycol (TEG) system and/or desiccants. By contrast, if anamine wash is used for CO₂ removal, water can be removed from the anodeexhaust downstream from the CO₂ removal stage.

Alternately or in addition to a CO₂ output stream and/or a water outputstream, the anode output can be used to form one or more product streamscontaining a desired chemical or fuel product. Such a product stream orstreams can correspond to a syngas stream, a hydrogen stream, or bothsyngas product and hydrogen product streams. For example, a hydrogenproduct stream containing at least about 70 vol % H₂, such as at leastabout 90 vol % H₂ or at least about 95 vol % H₂, can be formed.Additionally or alternately, a syngas stream containing at least about70 vol % of H₂ and CO combined, such as at least about 90 vol % of H₂and CO can be formed. The one or more product streams can have a gasvolume corresponding to at least about 75% of the combined H₂ and CO gasvolumes in the anode output, such as at least about 85% or at leastabout 90% of the combined H₂ and CO gas volumes. It is noted that therelative amounts of H₂ and CO in the products streams may differ fromthe H₂ to CO ratio in the anode output based on use of water gas shiftreaction stages to convert between the products.

In some aspects, it can be desirable to remove or separate a portion ofthe H₂ present in the anode output. For example, in some aspects the H₂to CO ratio in the anode exhaust can be at least about 3.0:1. Bycontrast, processes that make use of syngas, such as Fischer-Tropschsynthesis, may consume H₂ and CO in a different ratio, such as a ratiothat is closer to 2:1. One alternative can be to use a water gas shiftreaction to modify the content of the anode output to have an H₂ to COratio closer to a desired syngas composition. Another alternative can beto use a membrane separation to remove a portion of the H₂ present inthe anode output to achieve a desired ratio of H₂ and CO, or stillalternately to use a combination of membrane separation and water gasshift reactions. One advantage of using a membrane separation to removeonly a portion of the H₂ in the anode output can be that the desiredseparation can be performed under relatively mild conditions. Since onegoal can be to produce a retentate that still has a substantial H₂content, a permeate of high purity hydrogen can be generated by membraneseparation without requiring severe conditions. For example, rather thanhaving a pressure on the permeate side of the membrane of about 100 kPaaor less (such as ambient pressure), the permeate side can be at anelevated pressure relative to ambient while still having sufficientdriving force to perform the membrane separation. Additionally oralternately, a sweep gas such as methane can be used to provide adriving force for the membrane separation. This can reduce the purity ofthe H₂ permeate stream, but may be advantageous, depending on thedesired use for the permeate stream.

In various aspects of the invention, at least a portion of the anodeexhaust stream (preferably after separation of CO₂ and/or H₂O) can beused as a feed for a process external to the fuel cell and associatedreforming stages. In various aspects, the anode exhaust can have a ratioof H₂ to CO of about 1.5:1 to about 10:1, such as at least about 3.0:1,or at least about 4.0:1, or at least about 5.0:1. A syngas stream can begenerated or withdrawn from the anode exhaust. The anode exhaust gas,optionally after separation of CO₂ and/or H₂O, and optionally afterperforming a water gas shift reaction and/or a membrane separation toremove excess hydrogen, can correspond to a stream containingsubstantial portions of H₂ and/or CO. For a stream with a relatively lowcontent of CO, such as a stream where the ratio of H₂ to CO is at leastabout 3:1, the anode exhaust can be suitable for use as an H₂ feed.Examples of processes that could benefit from an H₂ feed can include,but are not limited to, refinery processes, an ammonia synthesis plant,or a turbine in a (different) power generation system, or combinationsthereof. Depending on the application, still lower CO₂ contents can bedesirable. For a stream with an H₂-to-CO ratio of less than about 2.2 to1 and greater than about 1.9 to 1, the stream can be suitable for use asa syngas feed. Examples of processes that could benefit from a syngasfeed can include, but are not limited to, a gas-to-liquids plant (suchas a plant using a Fischer-Tropsch process with a non-shifting catalyst)and/or a methanol synthesis plant. The amount of the anode exhaust usedas a feed for an external process can be any convenient amount.Optionally, when a portion of the anode exhaust is used as a feed for anexternal process, a second portion of the anode exhaust can be recycledto the anode input and/or recycled to the combustion zone for acombustion-powered generator.

The input streams useful for different types of Fischer-Tropschsynthesis processes can provide an example of the different types ofproduct streams that may be desirable to generate from the anode output.For a Fischer-Tropsch synthesis reaction system that uses a shiftingcatalyst, such as an iron-based catalyst, the desired input stream tothe reaction system can include CO₂ in addition to H₂ and CO. If asufficient amount of CO₂ is not present in the input stream, aFischer-Tropsch catalyst with water gas shift activity can consume CO inorder to generate additional CO₂, resulting in a syngas that can bedeficient in CO. For integration of such a Fischer-Tropsch process withan MCFC fuel cell, the separation stages for the anode output can beoperated to retain a desired amount of CO₂ (and optionally H₂O) in thesyngas product. By contrast, for a Fischer-Tropsch catalyst based on anon-shifting catalyst, any CO₂ present in a product stream could serveas an inert component in the Fischer-Tropsch reaction system.

In an aspect where the membrane is swept with a sweep gas such as amethane sweep gas, the methane sweep gas can correspond to a methanestream used as the anode fuel or in a different low pressure process,such as a boiler, furnace, gas turbine, or other fuel-consuming device.In such an aspect, low levels of CO₂ permeation across the membrane canhave minimal consequence. Such CO₂ that may permeate across the membranecan have a minimal impact on the reactions within the anode, and suchCO₂ can remain contained in the anode product. Therefore, the CO₂ (ifany) lost across the membrane due to permeation does not need to betransferred again across the MCFC electrolyte. This can significantlyreduce the separation selectivity requirement for the hydrogenpermeation membrane. This can allow, for example, use of ahigher-permeability membrane having a lower selectivity, which canenable use of a lower pressure and/or reduced membrane surface area. Insuch an aspect of the invention, the volume of the sweep gas can be alarge multiple of the volume of hydrogen in the anode exhaust, which canallow the effective hydrogen concentration on the permeate side to bemaintained close to zero. The hydrogen thus separated can beincorporated into the turbine-fed methane where it can enhance theturbine combustion characteristics, as described above.

It is noted that excess H₂ produced in the anode can represent a fuelwhere the greenhouse gases have already been separated. Any CO₂ in theanode output can be readily separated from the anode output, such as byusing an amine wash, a cryogenic CO₂ separator, and/or a pressure orvacuum swing absorption process. Several of the components of the anodeoutput (H₂, CO, CH₄) are not easily removed, while CO₂ and H₂O canusually be readily removed. Depending on the embodiment, at least about90 vol % of the CO₂ in the anode output can be separated out to form arelatively high purity CO₂ output stream. Thus, any CO₂ generated in theanode can be efficiently separated out to form a high purity CO₂ outputstream. After separation, the remaining portion of the anode output cancorrespond primarily to components with chemical and/or fuel value, aswell as reduced amounts of CO₂ and/or H₂O. Since a substantial portionof the CO₂ generated by the original fuel (prior to reforming) can havebeen separated out, the amount of CO₂ generated by subsequent burning ofthe remaining portion of the anode output can be reduced. In particular,to the degree that the fuel in the remaining portion of the anode outputis H₂, no additional greenhouse gases can typically be formed by burningof this fuel.

The anode exhaust can be subjected to a variety of gas processingoptions, including water-gas shift and separation of the components fromeach other. Two general anode processing schemes are shown in FIGS. 1and 2.

FIG. 1 schematically shows an example of a reaction system for operatinga fuel cell array of molten carbonate fuel cells in conjunction with achemical synthesis process. In FIG. 1, a fuel stream 105 is provided toa reforming stage (or stages) 110 associated with the anode 127 of afuel cell 120, such as a fuel cell that is part of a fuel cell stack ina fuel cell array. The reforming stage 110 associated with fuel cell 120can be internal to a fuel cell assembly. In some optional aspects, anexternal reforming stage (not shown) can also be used to reform aportion of the reformable fuel in an input stream prior to passing theinput stream into a fuel cell assembly. Fuel stream 105 can preferablyinclude a reformable fuel, such as methane, other hydrocarbons, and/orother hydrocarbon-like compounds such as organic compounds containingcarbon-hydrogen bonds. Fuel stream 105 can also optionally contain H₂and/or CO, such as H₂ and/or CO provided by optional anode recyclestream 185. It is noted that anode recycle stream 185 is optional, andthat in many aspects no recycle stream is provided from the anodeexhaust 125 back to anode 127, either directly or indirectly viacombination with fuel stream 105 or reformed fuel stream 115. Afterreforming, the reformed fuel stream 115 can be passed into anode 127 offuel cell 120. A CO₂ and O₂-containing stream 119 can also be passedinto cathode 129. A flow of carbonate ions 122, CO₃ ²⁻, from the cathodeportion 129 of the fuel cell can provide the remaining reactant neededfor the anode fuel cell reactions. Based on the reactions in the anode127, the resulting anode exhaust 125 can include H₂O, CO₂, one or morecomponents corresponding to incompletely reacted fuel (H₂, CO, CH₄, orother components corresponding to a reformable fuel), and optionally oneor more additional nonreactive components, such as N₂ and/or othercontaminants that are part of fuel stream 105. The anode exhaust 125 canthen be passed into one or more separation stages. For example, a CO₂removal stage 140 can correspond to a cryogenic CO₂ removal system, anamine wash stage for removal of acid gases such as CO₂, or anothersuitable type of CO₂ separation stage for separating a CO₂ output stream143 from the anode exhaust. Optionally, the anode exhaust can first bepassed through a water gas shift reactor 130 to convert any CO presentin the anode exhaust (along with some H₂O) into CO₂ and H₂ in anoptionally water gas shifted anode exhaust 135. Depending on the natureof the CO₂ removal stage, a water condensation or removal stage 150 maybe desirable to remove a water output stream 153 from the anode exhaust.Though shown in FIG. 1 after the CO₂ separation stage 140, it mayoptionally be located before the CO₂ separation stage 140 instead.Additionally, an optional membrane separation stage 160 for separationof H₂ can be used to generate a high purity permeate stream 163 of H₂.The resulting retentate stream 166 can then be used as an input to achemical synthesis process. Stream 166 could additionally or alternatelybe shifted in a second water-gas shift reactor 131 to adjust the H₂, CO,and CO₂ content to a different ratio, producing an output stream 168 forfurther use in a chemical synthesis process. In FIG. 1, anode recyclestream 185 is shown as being withdrawn from the retentate stream 166,but the anode recycle stream 185 could additionally or alternately bewithdrawn from other convenient locations in or between the variousseparation stages. The separation stages and shift reactor(s) couldadditionally or alternately be configured in different orders, and/or ina parallel configuration. Finally, a stream with a reduced content ofCO₂ 139 can be generated as an output from cathode 129. For the sake ofsimplicity, various stages of compression and heat addition/removal thatmight be useful in the process, as well as steam addition or removal,are not shown.

As noted above, the various types of separations performed on the anodeexhaust can be performed in any convenient order. FIG. 2 shows anexample of an alternative order for performing separations on an anodeexhaust. In FIG. 2, anode exhaust 125 can be initially passed intoseparation stage 260 for removing a portion 263 of the hydrogen contentfrom the anode exhaust 125. This can allow, for example, reduction ofthe H₂ content of the anode exhaust to provide a retentate 266 with aratio of H₂ to CO closer to 2:1. The ratio of H₂ to CO can then befurther adjusted to achieve a desired value in a water gas shift stage230. The water gas shifted output 235 can then pass through CO₂separation stage 240 and water removal stage 250 to produce an outputstream 275 suitable for use as an input to a desired chemical synthesisprocess. Optionally, output stream 275 could be exposed to an additionalwater gas shift stage (not shown). A portion of output stream 275 canoptionally be recycled (not shown) to the anode input. Of course, stillother combinations and sequencing of separation stages can be used togenerate a stream based on the anode output that has a desiredcomposition. For the sake of simplicity, various stages of compressionand heat addition/removal that might be useful in the process, as wellas steam addition or removal, are not shown.

Cathode Inputs and Outputs

Conventionally, a molten carbonate fuel cell can be operated based ondrawing a desired load while consuming some portion of the fuel in thefuel stream delivered to the anode. The voltage of the fuel cell canthen be determined by the load, fuel input to the anode, air and CO₂provided to the cathode, and the internal resistances of the fuel cell.The CO₂ to the cathode can be conventionally provided in part by usingthe anode exhaust as at least a part of the cathode input stream. Bycontrast, the present invention can use separate/different sources forthe anode input and cathode input. By removing any direct link betweenthe composition of the anode input flow and the cathode input flow,additional options become available for operating the fuel cell, such asto generate excess synthesis gas, to improve capture of carbon dioxide,and/or to improve the total efficiency (electrical plus chemical power)of the fuel cell, among others.

In a molten carbonate fuel cell, the transport of carbonate ions acrossthe electrolyte in the fuel cell can provide a method for transportingCO₂ from a first flow path to a second flow path, where the transportmethod can allow transport from a lower concentration (the cathode) to ahigher concentration (the anode), which can thus facilitate capture ofCO₂. Part of the selectivity of the fuel cell for CO₂ separation can bebased on the electrochemical reactions allowing the cell to generateelectrical power. For nonreactive species (such as N₂) that effectivelydo not participate in the electrochemical reactions within the fuelcell, there can be an insignificant amount of reaction and transportfrom cathode to anode. By contrast, the potential (voltage) differencebetween the cathode and anode can provide a strong driving force fortransport of carbonate ions across the fuel cell. As a result, thetransport of carbonate ions in the molten carbonate fuel cell can allowCO₂ to be transported from the cathode (lower CO₂ concentration) to theanode (higher CO₂ concentration) with relatively high selectivity.However, a challenge in using molten carbonate fuel cells for carbondioxide removal can be that the fuel cells have limited ability toremove carbon dioxide from relatively dilute cathode feeds. The voltageand/or power generated by a carbonate fuel cell can start to droprapidly as the CO₂ concentration falls below about 2.0 vol %. As the CO₂concentration drops further, e.g., to below about 1.0 vol %, at somepoint the voltage across the fuel cell can become low enough that littleor no further transport of carbonate may occur and the fuel cell ceasesto function. Thus, at least some CO₂ is likely to be present in theexhaust gas from the cathode stage of a fuel cell under commerciallyviable operating conditions.

The amount of carbon dioxide delivered to the fuel cell cathode(s) canbe determined based on the CO₂ content of a source for the cathodeinlet. One example of a suitable CO₂-containing stream for use as acathode input flow can be an output or exhaust flow from a combustionsource. Examples of combustion sources include, but are not limited to,sources based on combustion of natural gas, combustion of coal, and/orcombustion of other hydrocarbon-type fuels (including biologicallyderived fuels). Additional or alternate sources can include other typesof boilers, fired heaters, furnaces, and/or other types of devices thatburn carbon-containing fuels in order to heat another substance (such aswater or air). To a first approximation, the CO₂ content of the outputflow from a combustion source can be a minor portion of the flow. Evenfor a higher CO₂ content exhaust flow, such as the output from acoal-fired combustion source, the CO₂ content from most commercialcoal-fired power plants can be about 15 vol % or less. More generally,the CO₂ content of an output or exhaust flow from a combustion sourcecan be at least about 1.5 vol %, or at least about 1.6 vol %, or atleast about 1.7 vol %, or at least about 1.8 vol %, or at least about1.9 vol %, or at least greater 2 vol %, or at least about 4 vol %, or atleast about 5 vol %, or at least about 6 vol %, or at least about 8 vol%. Additionally or alternately, the CO₂ content of an output or exhaustflow from a combustion source can be about 20 vol % or less, such asabout 15 vol % or less, or about 12 vol % or less, or about 10 vol % orless, or about 9 vol % or less, or about 8 vol % or less, or about 7 vol% or less, or about 6.5 vol % or less, or about 6 vol % or less, orabout 5.5 vol % or less, or about 5 vol % or less, or about 4.5 vol % orless. The concentrations given above are on a dry basis. It is notedthat the lower CO₂ content values can be present in the exhaust fromsome natural gas or methane combustion sources, such as generators thatare part of a power generation system that may or may not include anexhaust gas recycle loop.

Other potential sources for a cathode input stream can additionally oralternately include sources of bio-produced CO₂. This can include, forexample, CO₂ generated during processing of bio-derived compounds, suchas CO₂ generated during ethanol production. An additional or alternateexample can include CO₂ generated by combustion of a bio-produced fuel,such as combustion of lignocellulose. Still other additional oralternate potential CO₂ sources can correspond to output or exhauststreams from various industrial processes, such as CO₂-containingstreams generated by plants for manufacture of steel, cement, and/orpaper.

Yet another additional or alternate potential source of CO₂ can beCO₂-containing streams from a fuel cell. The CO₂-containing stream froma fuel cell can correspond to a cathode output stream from a differentfuel cell, an anode output stream from a different fuel cell, a recyclestream from the cathode output to the cathode input of a fuel cell,and/or a recycle stream from an anode output to a cathode input of afuel cell. For example, an MCFC operated in standalone mode underconventional conditions can generate a cathode exhaust with a CO₂concentration of at least about 5 vol %. Such a CO₂-containing cathodeexhaust could be used as a cathode input for an MCFC operated accordingto an aspect of the invention. More generally, other types of fuel cellsthat generate a CO₂ output from the cathode exhaust can additionally oralternately be used, as well as other types of CO₂-containing streamsnot generated by a “combustion” reaction and/or by a combustion-poweredgenerator. Optionally but preferably, a CO₂-containing stream fromanother fuel cell can be from another molten carbonate fuel cell. Forexample, for molten carbonate fuel cells connected in series withrespect to the cathodes, the output from the cathode for a first moltencarbonate fuel cell can be used as the input to the cathode for a secondmolten carbonate fuel cell.

For various types of CO₂-containing streams from sources other thancombustion sources, the CO₂ content of the stream can vary widely. TheCO₂ content of an input stream to a cathode can contain at least about 2vol % of CO₂, such as at least about 4 vol %, or at least about 5 vol %,or at least about 6 vol %, or at least about 8 vol %. Additionally oralternately, the CO₂ content of an input stream to a cathode can beabout 30 vol % or less, such as about 25 vol % or less, or about 20 vol% or less, or about 15 vol % or less, or about 10 vol % or less, orabout 8 vol % or less, or about 6 vol % or less, or about 4 vol % orless. For some still higher CO₂ content streams, the CO₂ content can begreater than about 30 vol %, such as a stream substantially composed ofCO₂ with only incidental amounts of other compounds. As an example, agas-fired turbine without exhaust gas recycle can produce an exhauststream with a CO₂ content of approximately 4.2 vol %. With EGR, agas-fired turbine can produce an exhaust stream with a CO₂ content ofabout 6-8 vol %. Stoichiometric combustion of methane can produce anexhaust stream with a CO₂ content of about 11 vol %. Combustion of coalcan produce an exhaust stream with a CO₂ content of about 15-20 vol %.Fired heaters using refinery off-gas can produce an exhaust stream witha CO₂ content of about 12-15 vol %. A gas turbine operated on a low BTUgas without any EGR can produce an exhaust stream with a CO₂ content of˜12 vol %.

In addition to CO₂, a cathode input stream must include O₂ to providethe components necessary for the cathode reaction. Some cathode inputstreams can be based on having air as a component. For example, acombustion exhaust stream can be formed by combusting a hydrocarbon fuelin the presence of air. Such a combustion exhaust stream, or anothertype of cathode input stream having an oxygen content based on inclusionof air, can have an oxygen content of about 20 vol % or less, such asabout 15 vol % or less, or about 10 vol % or less. Additionally oralternately, the oxygen content of the cathode input stream can be atleast about 4 vol %, such as at least about 6 vol %, or at least about 8vol %. More generally, a cathode input stream can have a suitablecontent of oxygen for performing the cathode reaction. In some aspects,this can correspond to an oxygen content of about 5 vol % to about 15vol %, such as from about 7 vol % to about 9 vol %. For many types ofcathode input streams, the combined amount of CO₂ and O₂ can correspondto less than about 21 vol % of the input stream, such as less than about15 vol % of the stream or less than about 10 vol % of the stream. An airstream containing oxygen can be combined with a CO₂ source that has lowoxygen content. For example, the exhaust stream generated by burningcoal may include a low oxygen content that can be mixed with air to forma cathode inlet stream.

In addition to CO₂ and O₂, a cathode input stream can also be composedof inert/non-reactive species such as N₂, H₂O, and other typical oxidant(air) components. For example, for a cathode input derived from anexhaust from a combustion reaction, if air is used as part of theoxidant source for the combustion reaction, the exhaust gas can includetypical components of air such as N₂, H₂O, and other compounds in minoramounts that are present in air. Depending on the nature of the fuelsource for the combustion reaction, additional species present aftercombustion based on the fuel source may include one or more of H₂O,oxides of nitrogen (NOx) and/or sulfur (SOx), and other compounds eitherpresent in the fuel and/or that are partial or complete combustionproducts of compounds present in the fuel, such as CO. These species maybe present in amounts that do not poison the cathode catalyst surfacesthough they may reduce the overall cathode activity. Such reductions inperformance may be acceptable, or species that interact with the cathodecatalyst may be reduced to acceptable levels by known pollutant removaltechnologies.

The amount of O₂ present in a cathode input stream (such as an inputcathode stream based on a combustion exhaust) can advantageously besufficient to provide the oxygen needed for the cathode reaction in thefuel cell. Thus, the volume percentage of O₂ can advantageously be atleast 0.5 times the amount of CO₂ in the exhaust. Optionally, asnecessary, additional air can be added to the cathode input to providesufficient oxidant for the cathode reaction. When some form of air isused as the oxidant, the amount of N₂ in the cathode exhaust can be atleast about 78 vol %, e.g., at least about 88 vol %, and/or about 95 vol% or less. In some aspects, the cathode input stream can additionally oralternately contain compounds that are generally viewed as contaminants,such as H₂S or NH₃. In other aspects, the cathode input stream can becleaned to reduce or minimize the content of such contaminants.

In addition to the reaction to form carbonate ions for transport acrossthe electrolyte, the conditions in the cathode can also be suitable forconversion of nitrogen oxides into nitrate and/or nitrate ions.Hereinafter, only nitrate ions will be referred to for convenience. Theresulting nitrate ions can also be transported across the electrolytefor reaction in the anode. NOx concentrations in a cathode input streamcan typically be on the order of ppm, so this nitrate transport reactioncan have a minimal impact on the amount of carbonate transported acrossthe electrolyte. However, this method of NOx removal can be beneficialfor cathode input streams based on combustion exhausts from gasturbines, as this can provide a mechanism for reducing NOx emissions.The conditions in the cathode can additionally or alternately besuitable for conversion of unburned hydrocarbons (in combination with O₂in the cathode input stream) to typical combustion products, such as CO₂and H₂O.

A suitable temperature for operation of an MCFC can be between about450° C. and about 750° C., such as at least about 500° C., e.g., with aninlet temperature of about 550° C. and an outlet temperature of about625° C. Prior to entering the cathode, heat can be added to or removedfrom the combustion exhaust, if desired, e.g., to provide heat for otherprocesses, such as reforming the fuel input for the anode. For example,if the source for the cathode input stream is a combustion exhauststream, the combustion exhaust stream may have a temperature greaterthan a desired temperature for the cathode inlet. In such an aspect,heat can be removed from the combustion exhaust prior to use as thecathode input stream. Alternatively, the combustion exhaust could be atvery low temperature, for example after a wet gas scrubber on acoal-fired boiler, in which case the combustion exhaust can be belowabout 100° C. Alternatively, the combustion exhaust could be from theexhaust of a gas turbine operated in combined cycle mode, in which thegas can be cooled by raising steam to run a steam turbine for additionalpower generation. In this case, the gas can be below about 50° C. Heatcan be added to a combustion exhaust that is cooler than desired.

Fuel Cell Arrangement

In various aspects, a configuration option for a fuel cell (such as afuel cell array containing multiple fuel cell stacks) can be to dividethe CO₂-containing stream between a plurality of fuel cells. Some typesof sources for CO₂-containing streams can generate large volumetric flowrates relative to the capacity of an individual fuel cell. For example,the CO₂-containing output stream from an industrial combustion sourcecan typically correspond to a large flow volume relative to desirableoperating conditions for a single MCFC of reasonable size. Instead ofprocessing the entire flow in a single MCFC, the flow can be dividedamongst a plurality of MCFC units, usually at least some of which can bein parallel, so that the flow rate in each unit can be within a desiredflow range.

A second configuration option can be to utilize fuel cells in series tosuccessively remove CO₂ from a flow stream. Regardless of the number ofinitial fuel cells to which a CO₂-containing stream can be distributedto in parallel, each initial fuel cell can be followed by one or moreadditional cells in series to further remove additional CO₂. If thedesired amount of CO₂ in the cathode output is sufficiently low,attempting to remove CO₂ from a cathode input stream down to the desiredlevel in a single fuel cell or fuel cell stage could lead to a lowand/or unpredictable voltage output for the fuel cell. Rather thanattempting to remove CO₂ to the desired level in a single fuel cell orfuel cell stage, CO₂ can be removed in successive cells until a desiredlevel can be achieved. For example, each cell in a series of fuel cellscan be used to remove some percentage (e.g., about 50%) of the CO₂present in a fuel stream. In such an example, if three fuel cells areused in series, the CO₂ concentration can be reduced (e.g., to about 15%or less of the original amount present, which can correspond to reducingthe CO₂ concentration from about 6% to about 1% or less over the courseof three fuel cells in series).

In another configuration, the operating conditions can be selected inearly fuel stages in series to provide a desired output voltage whilethe array of stages can be selected to achieve a desired level of carbonseparation. As an example, an array of fuel cells can be used with threefuel cells in series. The first two fuel cells in series can be used toremove CO₂ while maintaining a desired output voltage. The final fuelcell can then be operated to remove CO₂ to a desired concentration butat a lower voltage.

In still another configuration, there can be separate connectivity forthe anodes and cathodes in a fuel cell array. For example, if the fuelcell array includes fuel cathodes connected in series, the correspondinganodes can be connected in any convenient manner, not necessarilymatching up with the same arrangement as their corresponding cathodes,for example. This can include, for instance, connecting the anodes inparallel, so that each anode receives the same type of fuel feed, and/orconnecting the anodes in a reverse series, so that the highest fuelconcentration in the anodes can correspond to those cathodes having thelowest CO₂ concentration.

In yet another configuration, the amount of fuel delivered to one ormore anode stages and/or the amount of CO₂ delivered to one or morecathode stages can be controlled in order to improve the performance ofthe fuel cell array. For example, a fuel cell array can have a pluralityof cathode stages connected in series. In an array that includes threecathode stages in series, this can mean that the output from a firstcathode stage can correspond to the input for a second cathode stage,and the output from the second cathode stage can correspond to the inputfor a third cathode stage. In this type of configuration, the CO₂concentration can decrease with each successive cathode stage. Tocompensate for this reduced CO₂ concentration, additional hydrogenand/or methane can be delivered to the anode stages corresponding to thelater cathode stages. The additional hydrogen and/or methane in theanodes corresponding to the later cathode stages can at least partiallyoffset the loss of voltage and/or current caused by the reduced CO₂concentration, which can increase the voltage and thus net powerproduced by the fuel cell. In another example, the cathodes in a fuelcell array can be connected partially in series and partially inparallel. In this type of example, instead of passing the entirecombustion output into the cathodes in the first cathode stage, at leasta portion of the combustion exhaust can be passed into a later cathodestage. This can provide an increased CO₂ content in a later cathodestage. Still other options for using variable feeds to either anodestages or cathode stages can be used if desired.

The cathode of a fuel cell can correspond to a plurality of cathodesfrom an array of fuel cells, as previously described. In some aspects, afuel cell array can be operated to improve or maximize the amount ofcarbon transferred from the cathode to the anode. In such aspects, forthe cathode output from the final cathode(s) in an array sequence(typically at least including a series arrangement, or else the finalcathode(s) and the initial cathode(s) would be the same), the outputcomposition can include about 2.0 vol % or less of CO₂ (e.g., about 1.5vol % or less or about 1.2 vol % or less) and/or at least about 0.5 vol% of CO₂, or at least about 1.0 vol %, or at least about 1.2 vol % or atleast about 1.5 vol %. Due to this limitation, the net efficiency of CO₂removal when using molten carbonate fuel cells can be dependent on theamount of CO₂ in the cathode input. For cathode input streams with CO₂contents of greater than about 6 vol %, such as at least about 8%, thelimitation on the amount of CO₂ that can be removed is not severe.However, for a combustion reaction using natural gas as a fuel and withexcess air, as is typically found in a gas turbine, the amount of CO₂ inthe combustion exhaust may only correspond to a CO₂ concentration at thecathode input of less than about 5 vol %. Use of exhaust gas recycle canallow the amount of CO₂ at the cathode input to be increased to at leastabout 5 vol %, e.g., at least about 6 vol %. If EGR is increased whenusing natural gas as a fuel to produce a CO₂ concentration beyond about6 vol %, then the flammability in the combustor can be decreased and thegas turbine may become unstable. However, when H₂ is added to the fuel,the flammability window can be significantly increased, allowing theamount of exhaust gas recycle to be increased further, so thatconcentrations of CO₂ at the cathode input of at least about 7.5 vol %or at least about 8 vol % can be achieved. As an example, based on aremoval limit of about 1.5 vol % at the cathode exhaust, increasing theCO₂ content at the cathode input from about 5.5 vol % to about 7.5 vol %can correspond to a ˜10% increase in the amount of CO₂ that can becaptured using a fuel cell and transported to the anode loop foreventual CO₂ separation. The amount of O₂ in the cathode output canadditionally or alternately be reduced, typically in an amountproportional to the amount of CO₂ removed, which can result in smallcorresponding increases in the amount(s) of the other(non-cathode-reactive) species at the cathode exit.

In other aspects, a fuel cell array can be operated to improve ormaximize the energy output of the fuel cell, such as the total energyoutput, the electric energy output, the syngas chemical energy output,or a combination thereof. For example, molten carbonate fuel cells canbe operated with an excess of reformable fuel in a variety ofsituations, such as for generation of a syngas stream for use inchemical synthesis plant and/or for generation of a high purity hydrogenstream. The syngas stream and/or hydrogen stream can be used as a syngassource, a hydrogen source, as a clean fuel source, and/or for any otherconvenient application. In such aspects, the amount of CO₂ in thecathode exhaust can be related to the amount of CO₂ in the cathode inputstream and the CO₂ utilization at the desired operating conditions forimproving or maximizing the fuel cell energy output.

Additionally or alternately, depending on the operating conditions, anMCFC can lower the CO₂ content of a cathode exhaust stream to about 5.0vol % or less, e.g., about 4.0 vol % or less, or about 2.0 vol % orless, or about 1.5 vol % or less, or about 1.2 vol % or less.Additionally or alternately, the CO₂ content of the cathode exhauststream can be at least about 0.9 vol %, such as at least about 1.0 vol%, or at least about 1.2 vol %, or at least about 1.5 vol %.

Molten Carbonate Fuel Cell Operation

In some aspects, a fuel cell may be operated in a single pass oronce-through mode. In single pass mode, reformed products in the anodeexhaust are not returned to the anode inlet. Thus, recycling syngas,hydrogen, or some other product from the anode output directly to theanode inlet is not done in single pass operation. More generally, insingle pass operation, reformed products in the anode exhaust are alsonot returned indirectly to the anode inlet, such as by using reformedproducts to process a fuel stream subsequently introduced into the anodeinlet. Optionally, CO₂ from the anode outlet can be recycled to thecathode inlet during operation of an MCFC in single pass mode. Moregenerally, in some alternative aspects, recycling from the anode outletto the cathode inlet may occur for an MCFC operating in single passmode. Heat from the anode exhaust or output may additionally oralternately be recycled in a single pass mode. For example, the anodeoutput flow may pass through a heat exchanger that cools the anodeoutput and warms another stream, such as an input stream for the anodeand/or the cathode. Recycling heat from anode to the fuel cell isconsistent with use in single pass or once-through operation. Optionallybut not preferably, constituents of the anode output may be burned toprovide heat to the fuel cell during single pass mode.

FIG. 3 shows a schematic example of the operation of an MCFC forgeneration of electrical power. In FIG. 3, the anode portion of the fuelcell can receive fuel and steam (H₂O) as inputs, with outputs of water,CO₂, and optionally excess H₂, CH₄ (or other hydrocarbons), and/or CO.The cathode portion of the fuel cell can receive CO₂ and some oxidant(e.g., air/O₂) as inputs, with an output corresponding to a reducedamount of CO₂ in O₂-depleted oxidant (air). Within the fuel cell, CO₃ ²⁻ions formed in the cathode side can be transported across theelectrolyte to provide the carbonate ions needed for the reactionsoccurring at the anode.

Several reactions can occur within a molten carbonate fuel cell such asthe example fuel cell shown in FIG. 3. The reforming reactions can beoptional, and can be reduced or eliminated if sufficient H₂ is provideddirectly to the anode. The following reactions are based on CH₄, butsimilar reactions can occur when other fuels are used in the fuel cell.

<anode reforming>CH₄+H₂O=>3H₂+CO  (1)

<water gas shift>CO+H₂O=>H₂+CO₂  (2)

<reforming and water gas shift combined>CH₄+2H₂O=>4H₂+CO₂  (3)

<anode H₂ oxidation>H₂+CO₃ ²⁻=>H₂O+CO₂+2e ⁻  (4)

<cathode>½O₂+CO₂+2e⁻=>CO₃ ²⁻  (5)

Reaction (1) represents the basic hydrocarbon reforming reaction togenerate H₂ for use in the anode of the fuel cell. The CO formed inreaction (1) can be converted to H₂ by the water-gas shift reaction (2).The combination of reactions (1) and (2) is shown as reaction (3).Reactions (1) and (2) can occur external to the fuel cell, and/or thereforming can be performed internal to the anode.

Reactions (4) and (5), at the anode and cathode respectively, representthe reactions that can result in electrical power generation within thefuel cell. Reaction (4) combines H₂, either present in the feed oroptionally generated by reactions (1) and/or (2), with carbonate ions toform H₂O, CO₂, and electrons to the circuit. Reaction (5) combines O₂,CO₂, and electrons from the circuit to form carbonate ions. Thecarbonate ions generated by reaction (5) can be transported across theelectrolyte of the fuel cell to provide the carbonate ions needed forreaction (4). In combination with the transport of carbonate ions acrossthe electrolyte, a closed current loop can then be formed by providingan electrical connection between the anode and cathode.

In various embodiments, a goal of operating the fuel cell can be toimprove the total efficiency of the fuel cell and/or the totalefficiency of the fuel cell plus an integrated chemical synthesisprocess. This is typically in contrast to conventional operation of afuel cell, where the goal can be to operate the fuel cell with highelectrical efficiency for using the fuel provided to the cell forgeneration of electrical power. As defined above, total fuel cellefficiency may be determined by dividing the electric output of the fuelcell plus the lower heating value of the fuel cell outputs by the lowerheating value of the input components for the fuel cell. In other words,TFCE=(LHV(el)+LHV(sg out))/LHV(in), where LHV(in) and LHV(sg out) referto the LHV of the fuel components (such as H₂, CH₄, and/or CO) deliveredto the fuel cell and syngas (H₂, CO and/or CO₂) in the anode outletstreams or flows, respectively. This can provide a measure of theelectric energy plus chemical energy generated by the fuel cell and/orthe integrated chemical process. It is noted that under this definitionof total efficiency, heat energy used within the fuel cell and/or usedwithin the integrated fuel cell/chemical synthesis system can contributeto total efficiency. However, any excess heat exchanged or otherwisewithdrawn from the fuel cell or integrated fuel cell/chemical synthesissystem is excluded from the definition. Thus, if excess heat from thefuel cell is used, for example, to generate steam for electricitygeneration by a steam turbine, such excess heat is excluded from thedefinition of total efficiency.

Several operational parameters may be manipulated to operate a fuel cellwith excess reformable fuel. Some parameters can be similar to thosecurrently recommended for fuel cell operation. In some aspects, thecathode conditions and temperature inputs to the fuel cell can besimilar to those recommended in the literature. For example, the desiredelectrical efficiency and the desired total fuel cell efficiency may beachieved at a range of fuel cell operating temperatures typical formolten carbonate fuel cells. In typical operation, the temperature canincrease across the fuel cell.

In other aspects, the operational parameters of the fuel cell candeviate from typical conditions so that the fuel cell is operated toallow a temperature decrease from the anode inlet to the anode outletand/or from the cathode inlet to the cathode outlet. For example, thereforming reaction to convert a hydrocarbon into H₂ and CO is anendothermic reaction. If a sufficient amount of reforming is performedin a fuel cell anode relative to the amount of oxidation of hydrogen togenerate electrical current, the net heat balance in the fuel cell canbe endothermic. This can cause a temperature drop between the inlets andoutlets of a fuel cell. During endothermic operation, the temperaturedrop in the fuel cell can be controlled so that the electrolyte in thefuel cell remains in a molten state.

Parameters that can be manipulated in a way so as to differ from thosecurrently recommended can include the amount of fuel provided to theanode, the composition of the fuel provided to the anode, and/or theseparation and capture of syngas in the anode output without significantrecycling of syngas from the anode exhaust to either the anode input orthe cathode input. In some aspects, no recycle of syngas or hydrogenfrom the anode exhaust to either the anode input or the cathode inputcan be allowed to occur, either directly or indirectly. In additional oralternative aspects, a limited amount of recycle can occur. In suchaspects, the amount of recycle from the anode exhaust to the anode inputand/or the cathode input can be less than about 10 vol % of the anodeexhaust, such as less than about 5 vol %, or less than about 1 vol %.

Additionally or alternately, a goal of operating a fuel cell can be toseparate CO₂ from the output stream of a combustion reaction or anotherprocess that produces a CO₂ output stream, in addition to allowinggeneration of electric power. In such aspects, the combustionreaction(s) can be used to power one or more generators or turbines,which can provide a majority of the power generated by the combinedgenerator/fuel cell system. Rather than operating the fuel cell tooptimize power generation by the fuel cell, the system can instead beoperated to improve the capture of carbon dioxide from thecombustion-powered generator while reducing or minimizing the number offuels cells required for capturing the carbon dioxide. Selecting anappropriate configuration for the input and output flows of the fuelcell, as well as selecting appropriate operating conditions for the fuelcell, can allow for a desirable combination of total efficiency andcarbon capture.

In some embodiments, the fuel cells in a fuel cell array can be arrangedso that only a single stage of fuel cells (such as fuel cell stacks) canbe present. In this type of embodiment, the anode fuel utilization forthe single stage can represent the anode fuel utilization for the array.Another option can be that a fuel cell array can contain multiple stagesof anodes and multiple stages of cathodes, with each anode stage havinga fuel utilization within the same range, such as each anode stagehaving a fuel utilization within 10% of a specified value, for examplewithin 5% of a specified value. Still another option can be that eachanode stage can have a fuel utilization equal to a specified value orlower than the specified value by less than an amount, such as havingeach anode stage be not greater than a specified value by 10% or less,for example, by 5% or less. As an illustrative example, a fuel cellarray with a plurality of anode stages can have each anode stage bewithin about 10% of 50% fuel utilization, which would correspond to eachanode stage having a fuel utilization between about 40% and about 60%.As another example, a fuel cell array with a plurality of stages canhave each anode stage be not greater than 60% anode fuel utilizationwith the maximum deviation being about 5% less, which would correspondto each anode stage having a fuel utilization between about 55% to about60%. In still another example, one or more stages of fuel cells in afuel cell array can be operated at a fuel utilization from about 30% toabout 50%, such as operating a plurality of fuel cell stages in thearray at a fuel utilization from about 30% to about 50%. More generally,any of the above types of ranges can be paired with any of the anodefuel utilization values specified herein.

Still another additional or alternate option can include specifying afuel utilization for less than all of the anode stages. For example, insome aspects of the invention fuel cells/stacks can be arranged at leastpartially in one or more series arrangements such that anode fuelutilization can be specified for the first anode stage in a series, thesecond anode stage in a series, the final anode stage in a series, orany other convenient anode stage in a series. As used herein, the“first” stage in a series corresponds to the stage (or set of stages, ifthe arrangement contains parallel stages as well) to which input isdirectly fed from the fuel source(s), with later (“second,” “third,”“final,” etc.) stages representing the stages to which the output fromone or more previous stages is fed, instead of directly from therespective fuel source(s). In situations where both output from previousstages and input directly from the fuel source(s) are co-fed into astage, there can be a “first” (set of) stage(s) and a “last” (set of)stage(s), but other stages (“second,” “third,” etc.) can be more trickyamong which to establish an order (e.g., in such cases, ordinal ordercan be determined by concentration levels of one or more components inthe composite input feed composition, such as CO₂ for instance, fromhighest concentration “first” to lowest concentration “last” withapproximately similar compositional distinctions representing the sameordinal level.)

Yet another additional or alternate option can be to specify the anodefuel utilization corresponding to a particular cathode stage (again,where fuel cells/stacks can be arranged at least partially in one ormore series arrangements). As noted above, based on the direction of theflows within the anodes and cathodes, the first cathode stage may notcorrespond to (be across the same fuel cell membrane from) the firstanode stage. Thus, in some aspects of the invention, the anode fuelutilization can be specified for the first cathode stage in a series,the second cathode stage in a series, the final cathode stage in aseries, or any other convenient cathode stage in a series.

Yet still another additional or alternate option can be to specify anoverall average of fuel utilization over all fuel cells in a fuel cellarray. In various aspects, the overall average of fuel utilization for afuel cell array can be about 65% or less, for example, about 60% orless, about 55% or less, about 50% or less, or about 45% or less(additionally or alternately, the overall average fuel utilization for afuel cell array can be at least about 25%, for example at least about30%, at least about 35%, or at least about 40%). Such an average fuelutilization need not necessarily constrain the fuel utilization in anysingle stage, so long as the array of fuel cells meets the desired fuelutilization.

Applications for CO₂ Output after Capture

In various aspects of the invention, the systems and methods describedabove can allow for production of carbon dioxide as a pressurized fluid.For example, the CO₂ generated from a cryogenic separation stage caninitially correspond to a pressurized CO₂ liquid with a purity of atleast about 90%, e.g., at least about 95%, at least about 97%, at leastabout 98%, or at least about 99%. This pressurized CO₂ stream can beused, e.g., for injection into wells in order to further enhance oil orgas recovery such as in secondary oil recovery. When done in proximityto a facility that encompasses a gas turbine, the overall system maybenefit from additional synergies in use of electrical/mechanical powerand/or through heat integration with the overall system.

Alternatively, for systems dedicated to an enhanced oil recovery (EOR)application (i.e., not comingled in a pipeline system with tightcompositional standards), the CO₂ separation requirements may besubstantially relaxed. The EOR application can be sensitive to thepresence of O₂, so O₂ can be absent, in some embodiments, from a CO₂stream intended for use in EOR. However, the EOR application can tend tohave a low sensitivity to dissolved CO, H₂, and/or CH₄. Also, pipelinesthat transport the CO₂ can be sensitive to these impurities. Thosedissolved gases can typically have only subtle impacts on thesolubilizing ability of CO₂ used for EOR. Injecting gases such as CO,H₂, and/or CH₄ as EOR gases can result in some loss of fuel valuerecovery, but such gases can be otherwise compatible with EORapplications.

Additionally or alternately, a potential use for CO₂ as a pressurizedliquid can be as a nutrient in biological processes such as algaegrowth/harvesting. The use of MCFCs for CO₂ separation can ensure thatmost biologically significant pollutants could be reduced to acceptablylow levels, resulting in a CO₂-containing stream having only minoramounts of other “contaminant” gases (such as CO, H₂, N₂, and the like,and combinations thereof) that are unlikely to substantially negativelyaffect the growth of photosynthetic organisms. This can be in starkcontrast to the output streams generated by most industrial sources,which can often contain potentially highly toxic material such as heavymetals.

In this type of aspect of the invention, the CO₂ stream generated byseparation of CO₂ in the anode loop can be used to produce biofuelsand/or chemicals, as well as precursors thereof. Further additionally oralternately, CO₂ may be produced as a dense fluid, allowing for mucheasier pumping and transport across distances, e.g., to large fields ofphotosynthetic organisms. Conventional emission sources can emit hot gascontaining modest amounts of CO₂ (e.g., about 4-15%) mixed with othergases and pollutants. These materials would normally need to be pumpedas a dilute gas to an algae pond or biofuel “farm”. By contrast, theMCFC system according to the invention can produce a concentrated CO₂stream (˜60-70% by volume on a dry basis) that can be concentratedfurther to 95%+(for example 96%+, 97%+, 98%+, or 99%+) and easilyliquefied. This stream can then be transported easily and efficientlyover long distances at relatively low cost and effectively distributedover a wide area. In these embodiments, residual heat from thecombustion source/MCFC may be integrated into the overall system aswell.

An alternative embodiment may apply where the CO₂ source/MCFC andbiological/chemical production sites are co-located. In that case, onlyminimal compression may be necessary (i.e., to provide enough CO₂pressure to use in the biological production, e.g., from about 15 psigto about 150 psig). Several novel arrangements can be possible in such acase. Secondary reforming may optionally be applied to the anode exhaustto reduce CH₄ content, and water-gas shift may optionally additionallyor alternately be present to drive any remaining CO into CO₂ and H₂.

The components from an anode output stream and/or cathode output streamcan be used for a variety of purposes. One option can be to use theanode output as a source of hydrogen, as described above. For an MCFCintegrated with or co-located with a refinery, the hydrogen can be usedas a hydrogen source for various refinery processes, such ashydroprocessing. Another option can be to additionally or alternatelyuse hydrogen as a fuel source where the CO₂ from combustion has alreadybeen “captured.” Such hydrogen can be used in a refinery or otherindustrial setting as a fuel for a boiler, furnace, and/or fired heater,and/or the hydrogen can be used as a feed for an electric powergenerator, such as a turbine. Hydrogen from an MCFC fuel cell canfurther additionally or alternately be used as an input stream for othertypes of fuel cells that require hydrogen as an input, possiblyincluding vehicles powered by fuel cells. Still another option can be toadditionally or alternately use syngas generated as an output from anMCFC fuel cell as a fermentation input.

Another option can be to additionally or alternately use syngasgenerated from the anode output. Of course, syngas can be used as afuel, although a syngas based fuel can still lead to some CO₂ productionwhen burned as fuel. In other aspects, a syngas output stream can beused as an input for a chemical synthesis process. One option can be toadditionally or alternately use syngas for a Fischer-Tropsch typeprocess, and/or another process where larger hydrocarbon molecules areformed from the syngas input. Another option can be to additionally oralternately use syngas to form an intermediate product such as methanol.Methanol could be used as the final product, but in other aspectsmethanol generated from syngas can be used to generate larger compounds,such as gasoline, olefins, aromatics, and/or other products. It is notedthat a small amount of CO₂ can be acceptable in the syngas feed to amethanol synthesis process, and/or to a Fischer-Tropsch processutilizing a shifting catalyst. Hydroformylation is an additional oralternate example of still another synthesis process that can make useof a syngas input.

It is noted that one variation on use of an MCFC to generate syngas canbe to use MCFC fuel cells as part of a system for processing methaneand/or natural gas withdrawn by an offshore oil platform or otherproduction system that is a considerable distance from its ultimatemarket. Instead of attempting to transport the gas phase output from awell, or attempting to store the gas phase product for an extendedperiod, the gas phase output from a well can be used as the input to anMCFC fuel cell array. This can lead to a variety of benefits. First, theelectric power generated by the fuel cell array can be used as a powersource for the platform. Additionally, the syngas output from the fuelcell array can be used as an input for a Fischer-Tropsch process at theproduction site. This can allow for formation of liquid hydrocarbonproducts more easily transported by pipeline, ship, or railcar from theproduction site to, for example, an on-shore facility or a largerterminal.

Still other integration options can additionally or alternately includeusing the cathode output as a source of higher purity, heated nitrogen.The cathode input can often include a large portion of air, which meansa substantial portion of nitrogen can be included in the cathode input.The fuel cell can transport CO₂ and O₂ from the cathode across theelectrolyte to the anode, and the cathode outlet can have lowerconcentrations of CO₂ and O₂, and thus a higher concentration of N₂ thanfound in air. With subsequent removal of the residual O₂ and CO₂, thisnitrogen output can be used as an input for production of ammonia orother nitrogen-containing chemicals, such as urea, ammonium nitrate,and/or nitric acid. It is noted that urea synthesis could additionallyor alternately use CO₂ separate from the anode output as an input feed.

Integration Example Applications for Integration with CombustionTurbines

In some aspects of the invention, a combustion source for generatingpower and exhausting a CO₂-containing exhaust can be integrated with theoperation of molten carbonate fuel cells. An example of a suitablecombustion source is a gas turbine. Preferably, the gas turbine cancombust natural gas, methane gas, or another hydrocarbon gas in acombined cycle mode integrated with steam generation and heat recoveryfor additional efficiency. Modern natural gas combined cycleefficiencies are about 60% for the largest and newest designs. Theresulting CO₂-containing exhaust gas stream can be produced at anelevated temperature compatible with the MCFC operation, such as 300°C.-700° C. and preferably 500° C.-650° C. The gas source can optionallybut preferably be cleaned of contaminants such as sulfur that can poisonthe MCFC before entering the turbine. Alternatively, the gas source canbe a coal-fired generator, wherein the exhaust gas would typically becleaned post-combustion due to the greater level of contaminants in theexhaust gas. In such an alternative, some heat exchange to/from the gasmay be necessary to enable clean-up at lower temperatures. In additionalor alternate embodiments, the source of the CO₂-containing exhaust gascan be the output from a boiler, combustor, or other heat source thatburns carbon-rich fuels. In other additional or alternate embodiments,the source of the CO₂-containing exhaust gas can be bio-produced CO₂ incombination with other sources.

For integration with a combustion source, some alternativeconfigurations for processing of a fuel cell anode can be desirable. Forexample, an alternative configuration can be to recycle at least aportion of the exhaust from a fuel cell anode to the input of a fuelcell anode. The output stream from an MCFC anode can include H₂O, CO₂,optionally CO, and optionally but typically unreacted fuel (such as H₂or CH₄) as the primary output components. Instead of using this outputstream as an external fuel stream and/or an input stream for integrationwith another process, one or more separations can be performed on theanode output stream in order to separate the CO₂ from the componentswith potential fuel value, such as H₂ or CO. The components with fuelvalue can then be recycled to the input of an anode.

This type of configuration can provide one or more benefits. First, CO₂can be separated from the anode output, such as by using a cryogenic CO₂separator. Several of the components of the anode output (H₂, CO, CH₄)are not easily condensable components, while CO₂ and H₂O can beseparated individually as condensed phases. Depending on the embodiment,at least about 90 vol % of the CO₂ in the anode output can be separatedto form a relatively high purity CO₂ output stream. Alternatively, insome aspects less CO₂ can be removed from the anode output, so thatabout 50 vol % to about 90 vol % of the CO₂ in the anode output can beseparated out, such as about 80 vol % or less or about 70 vol % or less.After separation, the remaining portion of the anode output cancorrespond primarily to components with fuel value, as well as reducedamounts of CO₂ and/or H₂O. This portion of the anode output afterseparation can be recycled for use as part of the anode input, alongwith additional fuel. In this type of configuration, even though thefuel utilization in a single pass through the MCFC(s) may be low, theunused fuel can be advantageously recycled for another pass through theanode. As a result, the single-pass fuel utilization can be at a reducedlevel, while avoiding loss (exhaust) of unburned fuel to theenvironment.

Additionally or alternatively to recycling a portion of the anodeexhaust to the anode input, another configuration option can be to use aportion of the anode exhaust as an input for a combustion reaction for aturbine or other combustion device, such as a boiler, furnace, and/orfired heater. The relative amounts of anode exhaust recycled to theanode input and/or as an input to the combustion device can be anyconvenient or desirable amount. If the anode exhaust is recycled to onlyone of the anode input and the combustion device, the amount of recyclecan be any convenient amount, such as up to 100% of the portion of theanode exhaust remaining after any separation to remove CO₂ and/or H₂O.When a portion of the anode exhaust is recycled to both the anode inputand the combustion device, the total recycled amount by definition canbe 100% or less of the remaining portion of anode exhaust. Otherwise,any convenient split of the anode exhaust can be used. In variousembodiments of the invention, the amount of recycle to the anode inputcan be at least about 10% of the anode exhaust remaining afterseparations, for example at least about 25%, at least about 40%, atleast about 50%, at least about 60%, at least about 75%, or at leastabout 90%. Additionally or alternately in those embodiments, the amountof recycle to the anode input can be about 90% or less of the anodeexhaust remaining after separations, for example about 75% or less,about 60% or less, about 50% or less, about 40% or less, about 25% orless, or about 10% or less. Further additionally or alternately, invarious embodiments of the invention, the amount of recycle to thecombustion device can be at least about 10% of the anode exhaustremaining after separations, for example at least about 25%, at leastabout 40%, at least about 50%, at least about 60%, at least about 75%,or at least about 90%. Additionally or alternately in those embodiments,the amount of recycle to the combustion device can be about 90% or lessof the anode exhaust remaining after separations, for example about 75%or less, about 60% or less, about 50% or less, about 40% or less, about25% or less, or about 10% or less.

In still other alternative aspects of the invention, the fuel for acombustion device can additionally or alternately be a fuel with anelevated quantity of components that are inert and/or otherwise act as adiluent in the fuel. CO₂ and N₂ are examples of components in a naturalgas feed that can be relatively inert during a combustion reaction. Whenthe amount of inert components in a fuel feed reaches a sufficientlevel, the performance of a turbine or other combustion source can beimpacted. The impact can be due in part to the ability of the inertcomponents to absorb heat, which can tend to quench the combustionreaction. Examples of fuel feeds with a sufficient level of inertcomponents can include fuel feeds containing at least about 20 vol %CO₂, or fuel feeds containing at least about 40 vol % N₂, or fuel feedscontaining combinations of CO₂ and N₂ that have sufficient inert heatcapacity to provide similar quenching ability. (It is noted that CO₂ hasa greater heat capacity than N₂, and therefore lower concentrations ofCO₂ can have a similar impact as higher concentrations of N₂. CO₂ canalso participate in the combustion reactions more readily than N₂, andin doing so remove H₂ from the combustion. This consumption of H₂ canhave a large impact on the combustion of the fuel, by reducing the flamespeed and narrowing the flammability range of the air and fuel mixture.)More generally, for a fuel feed containing inert components that impactthe flammability of the fuel feed, the inert components in the fuel feedcan be at least about 20 vol %, such as at least about 40 vol %, or atleast about 50 vol %, or at least about 60 vol %. Preferably, the amountof inert components in the fuel feed can be about 80 vol % or less.

When a sufficient amount of inert components are present in a fuel feed,the resulting fuel feed can be outside of the flammability window forthe fuel components of the feed. In this type of situation, addition ofH₂ from a recycled portion of the anode exhaust to the combustion zonefor the generator can expand the flammability window for the combinationof fuel feed and H₂, which can allow, for example, a fuel feedcontaining at least about 20 vol % CO₂ or at least about 40% N₂ (orother combinations of CO₂ and N₂) to be successfully combusted.

Relative to a total volume of fuel feed and H₂ delivered to a combustionzone, the amount of H₂ for expanding the flammability window can be atleast about 5 vol % of the total volume of fuel feed plus H₂, such as atleast about 10 vol %, and/or about 25 vol % or less. Another option forcharacterizing the amount of H₂ to add to expand the flammability windowcan be based on the amount of fuel components present in the fuel feedbefore H₂ addition. Fuel components can correspond to methane, naturalgas, other hydrocarbons, and/or other components conventionally viewedas fuel for a combustion-powered turbine or other generator. The amountof H₂ added to the fuel feed can correspond to at least about one thirdof the volume of fuel components (1:3 ratio of H₂:fuel component) in thefuel feed, such as at least about half of the volume of the fuelcomponents (1:2 ratio). Additionally or alternately, the amount of H₂added to the fuel feed can be roughly equal to the volume of fuelcomponents in the fuel feed (1:1 ratio) or less. For example, for a feedcontaining about 30 vol % CH₄, about 10% N₂, and about 60% CO₂, asufficient amount of anode exhaust can be added to the fuel feed toachieve about a 1:2 ratio of H₂ to CH₄. For an idealized anode exhaustthat contained only H₂, addition of H₂ to achieve a 1:2 ratio wouldresult in a feed containing about 26 vol % CH₄, 13 vol % H₂, 9 vol % N₂,and 52 vol % CO₂.

Exhaust Gas Recycle

Aside from providing exhaust gas to a fuel cell array for capture andeventual separation of the CO₂, an additional or alternate potential usefor exhaust gas can include recycle back to the combustion reaction toincrease the CO₂ content. When hydrogen is available for addition to thecombustion reaction, such as hydrogen from the anode exhaust of the fuelcell array, further benefits can be gained from using recycled exhaustgas to increase the CO₂ content within the combustion reaction.

In various aspects of the invention, the exhaust gas recycle loop of apower generation system can receive a first portion of the exhaust gasfrom combustion, while the fuel cell array can receive a second portion.The amount of exhaust gas from combustion recycled to the combustionzone of the power generation system can be any convenient amount, suchas at least about 15% (by volume), for example at least about 25%, atleast about 35%, at least about 45%, or at least about 50%. Additionallyor alternately, the amount of combustion exhaust gas recirculated to thecombustion zone can be about 65% (by volume) or less, e.g., about 60% orless, about 55% or less, about 50% or less, or about 45% or less.

In one or more aspects of the invention, a mixture of an oxidant (suchas air and/or oxygen-enriched air) and fuel can be combusted and(simultaneously) mixed with a stream of recycled exhaust gas. The streamof recycled exhaust gas, which can generally include products ofcombustion such as CO₂, can be used as a diluent to control, adjust, orotherwise moderate the temperature of combustion and of the exhaust thatcan enter the succeeding expander. As a result of using oxygen-enrichedair, the recycled exhaust gas can have an increased CO₂ content, therebyallowing the expander to operate at even higher expansion ratios for thesame inlet and discharge temperatures, thereby enabling significantlyincreased power production.

A gas turbine system can represent one example of a power generationsystem where recycled exhaust gas can be used to enhance the performanceof the system. The gas turbine system can have a first/main compressorcoupled to an expander via a shaft. The shaft can be any mechanical,electrical, or other power coupling, thereby allowing a portion of themechanical energy generated by the expander to drive the maincompressor. The gas turbine system can also include a combustion chamberconfigured to combust a mixture of a fuel and an oxidant. In variousaspects of the invention, the fuel can include any suitable hydrocarbongas/liquid, such as syngas, natural gas, methane, ethane, propane,butane, naphtha diesel, kerosene, aviation fuel, coal derived fuel,bio-fuel, oxygenated hydrocarbon feedstock, or any combinations thereof.The oxidant can, in some embodiments, be derived from a second or inletcompressor fluidly coupled to the combustion chamber and adapted tocompress a feed oxidant. In one or more embodiments of the invention,the feed oxidant can include atmospheric air and/or enriched air. Whenthe oxidant includes enriched air alone or a mixture of atmospheric airand enriched air, the enriched air can be compressed by the inletcompressor (in the mixture, either before or after being mixed with theatmospheric air). The enriched air and/or the air-enriched air mixturecan have an overall oxygen concentration of at least about 25 volume %,e.g., at least about 30 volume %, at least about 35 volume %, at leastabout 40 volume %, at least about 45 volume %, or at least about 50volume %. Additionally or alternately, the enriched air and/or theair-enriched air mixture can have an overall oxygen concentration ofabout 80 volume % or less, such as about 70 volume % or less.

The enriched air can be derived from any one or more of several sources.For example, the enriched air can be derived from such separationtechnologies as membrane separation, pressure swing adsorption,temperature swing adsorption, nitrogen plant-byproduct streams, and/orcombinations thereof. The enriched air can additionally or alternatelybe derived from an air separation unit (ASU), such as a cryogenic ASU,for producing nitrogen for pressure maintenance or other purposes. Incertain embodiments of the invention, the reject stream from such an ASUcan be rich in oxygen, having an overall oxygen content from about 50volume % to about 70 volume %, can be used as at least a portion of theenriched air and subsequently diluted, if needed, with unprocessedatmospheric air to obtain the desired oxygen concentration.

In addition to the fuel and oxidant, the combustion chamber canoptionally also receive a compressed recycle exhaust gas, such as anexhaust gas recirculation primarily having CO₂ and nitrogen components.The compressed recycle exhaust gas can be derived from the maincompressor, for instance, and adapted to help facilitate combustion ofthe oxidant and fuel, e.g., by moderating the temperature of thecombustion products. As can be appreciated, recirculating the exhaustgas can serve to increase CO₂ concentration.

An exhaust gas directed to the inlet of the expander can be generated asa product of combustion reaction. The exhaust gas can have a heightenedCO₂ content based, at least in part, on the introduction of recycledexhaust gas into the combustion reaction. As the exhaust gas expandsthrough the expander, it can generate mechanical power to drive the maincompressor, to drive an electrical generator, and/or to power otherfacilities.

The power generation system can, in many embodiments, also include anexhaust gas recirculation (EGR) system. In one or more aspects of theinvention, the EGR system can include a heat recovery steam generator(HRSG) and/or another similar device fluidly coupled to a steam gasturbine. In at least one embodiment, the combination of the HRSG and thesteam gas turbine can be characterized as a power-producing closedRankine cycle. In combination with the gas turbine system, the HRSG andthe steam gas turbine can form part of a combined-cycle power generatingplant, such as a natural gas combined-cycle (NGCC) plant. The gaseousexhaust can be introduced to the HRSG in order to generate steam and acooled exhaust gas. The HRSG can include various units for separatingand/or condensing water out of the exhaust stream, transferring heat toform steam, and/or modifying the pressure of streams to a desired level.In certain embodiments, the steam can be sent to the steam gas turbineto generate additional electrical power.

After passing through the HRSG and optional removal of at least someH₂O, the CO₂-containing exhaust stream can, in some embodiments, berecycled for use as an input to the combustion reaction. As noted above,the exhaust stream can be compressed (or decompressed) to match thedesired reaction pressure within the vessel for the combustion reaction.

Example of Integrated System

FIG. 4 schematically shows an example of an integrated system includingintroduction of both CO₂-containing recycled exhaust gas and H₂ or COfrom the fuel cell anode exhaust into the combustion reaction forpowering a turbine. In FIG. 4, the turbine can include a compressor 402,a shaft 404, an expander 406, and a combustion zone 415. An oxygensource 411 (such as air and/or oxygen-enriched air) can be combined withrecycled exhaust gas 498 and compressed in compressor 402 prior toentering combustion zone 415. A fuel 412, such as CH₄, and optionally astream containing H₂ or CO 187 can be delivered to the combustion zone.The fuel and oxidant can be reacted in zone 415 and optionally butpreferably passed through expander 406 to generate electric power. Theexhaust gas from expander 106 can be used to form two streams, e.g., aCO₂-containing stream 422 (that can be used as an input feed for fuelcell array 425) and another CO₂-containing stream 492 (that can be usedas the input for a heat recovery and steam generator system 490, whichcan, for example, enable additional electricity to be generated usingsteam turbines 494). After passing through heat recovery system 490,including optional removal of a portion of H₂O from the CO₂-containingstream, the output stream 498 can be recycled for compression incompressor 402 or a second compressor that is not shown. The proportionof the exhaust from expander 406 used for CO₂-containing stream 492 canbe determined based on the desired amount of CO₂ for addition tocombustion zone 415.

As used herein, the EGR ratio is the flow rate for the fuel cell boundportion of the exhaust gas divided by the combined flow rate for thefuel cell bound portion and the recovery bound portion, which is sent tothe heat recovery generator. For example, the EGR ratio for flows shownin FIG. 4 is the flow rate of stream 422 divided by the combined flowrate of streams 422 and 492.

The CO₂-containing stream 422 can be passed into a cathode portion (notshown) of a molten carbonate fuel cell array 425. Based on the reactionswithin fuel cell array 425, CO₂ can be separated from stream 422 andtransported to the anode portion (not shown) of the fuel cell array 425.This can result in a cathode output stream 424 depleted in CO₂. Thecathode output stream 424 can then be passed into a heat recovery (andoptional steam generator) system 450 for generation of heat exchangeand/or additional generation of electricity using steam turbines 454(which may optionally be the same as the aforementioned steam turbines494). After passing through heat recovery and steam generator system450, the resulting flue gas stream 456 can be exhausted to theenvironment and/or passed through another type of carbon capturetechnology, such as an amine scrubber.

After transport of CO₂ from the cathode side to the anode side of fuelcell array 425, the anode output 435 can optionally be passed into awater gas shift reactor 470. Water gas shift reactor 470 can be used togenerate additional H₂ and CO₂ at the expense of CO (and H₂O) present inthe anode output 435. The output from the optional water gas shiftreactor 470 can then be passed into one or more separation stages 440,such as a cold box or a cryogenic separator. This can allow forseparation of an H₂O stream 447 and CO₂ stream 449 from the remainingportion of the anode output. The remaining portion of the anode output485 can include unreacted H₂ generated by reforming but not consumed infuel cell array 425. A first portion 445 of the H₂-containing stream 485can be recycled to the input for the anode(s) in fuel cell array 425. Asecond portion 487 of stream 485 can be used as an input for combustionzone 415. A third portion 465 can be used as is for another purposeand/or treated for subsequent further use. Although FIG. 4 and thedescription herein schematically details up to three portions, it iscontemplated that only one of these three portions can be exploited,only two can be exploited, or all three can be exploited according tothe invention.

In FIG. 4, the exhaust for the exhaust gas recycle loop is provided by afirst heat recovery and steam generator system 490, while a second heatrecovery and steam generator system 450 can be used to capture excessheat from the cathode output of the fuel cell array 425. FIG. 5 shows analternative embodiment where the exhaust gas recycle loop is provided bythe same heat recovery steam generator used for processing the fuel cellarray output. In FIG. 5, recycled exhaust gas 598 is provided by heatrecovery and steam generator system 550 as a portion of the flue gasstream 556. This can eliminate the separate heat recovery and steamgenerator system associated with the turbine.

In various embodiments of the invention, the process can be approachedas starting with a combustion reaction for powering a turbine, aninternal combustion engine, or another system where heat and/or pressuregenerated by a combustion reaction can be converted into another form ofpower. The fuel for the combustion reaction can comprise or be hydrogen,a hydrocarbon, and/or any other compound containing carbon that can beoxidized (combusted) to release energy. Except for when the fuelcontains only hydrogen, the composition of the exhaust gas from thecombustion reaction can have a range of CO₂ contents, depending on thenature of the reaction (e.g., from at least about 2 vol % to about 25vol % or less). Thus, in certain embodiments where the fuel iscarbonaceous, the CO₂ content of the exhaust gas can be at least about 2vol %, for example at least about 4 vol %, at least about 5 vol %, atleast about 6 vol %, at least about 8 vol %, or at least about 10 vol %.Additionally or alternately in such carbonaceous fuel embodiments, theCO₂ content can be about 25 vol % or less, for example about 20 vol % orless, about 15 vol % or less, about 10 vol % or less, about 7 vol % orless, or about 5 vol % or less. Exhaust gases with lower relative CO₂contents (for carbonaceous fuels) can correspond to exhaust gases fromcombustion reactions on fuels such as natural gas with lean (excess air)combustion. Higher relative CO₂ content exhaust gases (for carbonaceousfuels) can correspond to optimized natural gas combustion reactions,such as those with exhaust gas recycle, and/or combustion of fuels suchas coal.

In some aspects of the invention, the fuel for the combustion reactioncan contain at least about 90 volume % of compounds containing fivecarbons or less, e.g., at least about 95 volume %. In such aspects, theCO₂ content of the exhaust gas can be at least about 4 vol %, forexample at least about 5 vol %, at least about 6 vol %, at least about 7vol %, or at least about 7.5 vol %. Additionally or alternately, the CO₂content of the exhaust gas can be about 13 vol % or less, e.g., about 12vol % or less, about 10 vol % or less, about 9 vol % or less, about 8vol % or less, about 7 vol % or less, or about 6 vol % or less. The CO₂content of the exhaust gas can represent a range of values depending onthe configuration of the combustion-powered generator. Recycle of anexhaust gas can be beneficial for achieving a CO₂ content of at leastabout 6 vol %, while addition of hydrogen to the combustion reaction canallow for further increases in CO₂ content to achieve a CO₂ content ofat least about 7.5 vol %.

Alternative Configuration—High Severity NOx Turbine

Gas turbines can be limited in their operation by several factors. Onetypical limitation can be that the maximum temperature in the combustionzone can be controlled below certain limits to achieve sufficiently lowconcentrations of nitrogen oxides (NOx) in order to satisfy regulatoryemission limits. Regulatory emission limits can require a combustionexhaust to have a NOx content of about 20 vppm or less, and possible 10vppm or less, when the combustion exhaust is allowed to exit to theenvironment.

NOx formation in natural gas-fired combustion turbines can be a functionof temperature and residence time. Reactions that result in formation ofNOx can be of reduced and/or minimal importance below a flametemperature of about 1500° F., but NOx production can increase rapidlyas the temperature increases beyond this point. In a gas turbine,initial combustion products can be mixed with extra air to cool themixture to a temperature around 1200° F., and temperature can be limitedby the metallurgy of the expander blades. Early gas turbines typicallyexecuted the combustion in diffusion flames that had stoichiometriczones with temperatures well above 1500° F., resulting in higher NOxconcentrations. More recently, the current generation of ‘Dry Low Nox’(DLN) burners can use special pre-mixed burners to burn natural gas atcooler lean (less fuel than stoichiometric) conditions. For example,more of the dilution air can be mixed in to the initial flame, and lesscan be mixed in later to bring the temperature down to the ˜1200° F.turbine-expander inlet temperature. The disadvantages for DLN burnerscan include poor performance at turndown, higher maintenance, narrowranges of operation, and poor fuel flexibility. The latter can be aconcern, as DLN burners can be more difficult to apply to fuels ofvarying quality (or difficult to apply at all to liquid fuels). For lowBTU fuels, such as fuels containing a high content of CO₂, DLN burnersare typically not used and instead diffusion burners can be used. Inaddition, gas turbine efficiency can be increased by using a higherturbine-expander inlet temperature. However, because there can be alimited amount of dilution air, and this amount can decrease withincreased turbine-expander inlet temperature, the DLN burner can becomeless effective at maintaining low NOx as the efficiency of the gasturbine improves.

In various aspects of the invention, a system integrating a gas turbinewith a fuel cell for carbon capture can allow use of higher combustionzone temperatures while reducing and/or minimizing additional NOxemissions, as well as enabling DLN-like NOx savings via use of turbinefuels that are not presently compatible with DLN burners. In suchaspects, the turbine can be run at higher power (i.e., highertemperature) resulting in higher NOx emissions, but also higher poweroutput and potentially higher efficiency. In some aspects of theinvention, the amount of NOx in the combustion exhaust can be at leastabout 20 vppm, such as at least about 30 vppm, or at least about 40vppm. Additionally or alternately, the amount of NOx in the combustionexhaust can be about 1000 vppm or less, such as about 500 vppm or less,or about 250 vppm or less, or about 150 vppm or less, or about 100 vppmor less. In order to reduce the NOx levels to levels required byregulation, the resulting NOx can be equilibrated via thermal NOxdestruction (reduction of NOx levels to equilibrium levels in theexhaust stream) through one of several mechanisms, such as simplethermal destruction in the gas phase; catalyzed destruction from thenickel cathode catalyst in the fuel cell array; and/or assisted thermaldestruction prior to the fuel cell by injection of small amounts ofammonia, urea, or other reductant. This can be assisted by introductionof hydrogen derived from the anode exhaust. Further reduction of NOx inthe cathode of the fuel cell can be achieved via electrochemicaldestruction wherein the NOx can react at the cathode surface and can bedestroyed. This can result in some nitrogen transport across themembrane electrolyte to the anode, where it may form ammonia or otherreduced nitrogen compounds. With respect to NOx reduction methodsinvolving an MCFC, the expected NOx reduction from a fuel cell/fuel cellarray can be about 80% or less of the NOx in the input to the fuel cellcathode, such as about 70% or less, and/or at least about 5%. It isnoted that sulfidic corrosion can also limit temperatures and affectturbine blade metallurgy in conventional systems. However, the sulfurrestrictions of the MCFC system can typically require reduced fuelsulfur levels that reduce or minimize concerns related to sulfidiccorrosion. Operating the MCFC array at low fuel utilization can furthermitigate such concerns, such as in aspects where a portion of the fuelfor the combustion reaction corresponds to hydrogen from the anodeexhaust.

ADDITIONAL EMBODIMENTS Embodiment 1

A method for producing electricity, the method comprising: introducing afuel stream comprising a reformable fuel into an anode of a moltencarbonate fuel cell, an internal reforming element associated with theanode, or a combination thereof; introducing a cathode inlet streamcomprising CO₂ and O₂ into a cathode of the molten carbonate fuel cell;generating electricity within the molten carbonate fuel cell, the moltencarbonate fuel cell being operated at a fuel utilization of about 60% orless; generating an anode exhaust comprising H₂, CO, and CO₂;separating, from at least a portion of the anode exhaust, a firstH₂-rich gas stream comprising at least about 80 vol % (e.g., at leastabout 90%) H₂; and combusting at least a portion of the first H₂-richgas stream to produce electricity.

Embodiment 2

The method of embodiment 1, further comprising (i) performing a watergas shift process on the anode exhaust, the at least a portion of theanode exhaust, or a combination thereof; (ii) separating CO₂ and/or H₂Ofrom the anode exhaust, the at least a portion of the anode exhaust, ora combination thereof; or (iii) both (i) and (ii).

Embodiment 3

The method of embodiment 1 or 2, wherein the separating step comprises:performing a water gas shift process on the anode exhaust or at least aportion of the anode exhaust to form a shifted anode exhaust portion;and separating H₂O and CO₂ from the shifted anode exhaust portion toform the first H₂-rich gas stream.

Embodiment 4

The method of any of the above embodiments, wherein combusting stepcomprises generating steam from heat generated by the combustion, andproducing electricity from at least a portion of the generated steam.

Embodiment 5

The method of any of the above embodiments, wherein the combusting stepcomprises combusting the at least a portion of the first H₂-rich gasstream in a turbine.

Embodiment 6

The method of any of the above embodiments, wherein the cathode inletstream comprises exhaust from combustion of a carbon-containing fuel ina combustion turbine.

Embodiment 7

The method of embodiment 6, wherein the carbon-containing fuel comprisesone or more of: at least 5 vol % of inert gases; at least about 10 vol %CO₂; and at least about 10 vol % N₂.

Embodiment 8

The method of any of the above embodiments, wherein the anode exhausthas a ratio of H₂:CO of at least about 2.5:1 (e.g., at least about3.0:1, at least about 4.0:1, or at least about 5.0:1).

Embodiment 9

The method of any of the above embodiments, further comprising forming asecond H₂-containing stream from the anode exhaust, the at least aportion of the anode exhaust, the first H₂-rich gas stream, or acombination thereof; and recycling at least a portion of the secondH₂-containing stream to the combustion turbine.

Embodiment 10

The method of any of the above embodiments, wherein at least about 90vol % of the reformable fuel is methane.

Embodiment 11

The method of any of the above embodiments, wherein the molten carbonatefuel cell is operated at a thermal ratio from about 0.25 to about 1.5(e.g., from about 0.25 to about 1.3, from about 0.25 to about 1.15, fromabout 0.25 to about 1.0, from about 0.25 to about 0.85, from about 0.25to about 0.8, or from about 0.25 to about 0.75).

Embodiment 12

The method of any of the above embodiments, wherein an amount of thereformable fuel introduced into the anode, the internal reformingelement associated with the anode, or the combination thereof, is atleast about 50% greater (e.g., at least about 75% greater or at leastabout 100% greater) than an amount of hydrogen reacted in the moltencarbonate fuel cell to generate electricity.

Embodiment 13

The method of any of the above embodiments, wherein a ratio of net molesof syngas in the anode exhaust to moles of CO₂ in a cathode exhaust isat least about 2.0:1 (e.g., at least about 3.0, at least about 4.0, atleast about 5.0, at least about 10.0, or at least about 20.0), andoptionally about 40.0 or less (e.g., about 30.0 or less or about 20.0 orless).

Embodiment 14

The method of any of the above embodiments, wherein a fuel utilizationin the anode is about 50% or less (e.g., about 30% or less, about 25% orless, or about 20% or less) and a CO₂ utilization in the cathode is atleast about 60% (e.g., at least about 65%, at least about 70%, or atleast about 75%).

Embodiment 15

The method of any of the above embodiments, wherein the molten carbonatefuel cell is operated to generate electrical power at a current densityof at least about 150 mA/cm² and at least about 40 mW/cm² (e.g., atleast about 50 mW/cm², at least about 60 mW/cm², at least about 80mW/cm², or at least 100 mW/cm²) of waste heat, the method furthercomprising performing an effective amount of an endothermic reaction tomaintain a temperature differential between an anode inlet and an anodeoutlet of about 100° C. or less (e.g., about 80° C. or less or about 60°C. or less), and optionally wherein performing the endothermic reactionconsumes at least about 40% (e.g., at least about 50%, at least about60%, or at least about 75%) of the waste heat.

Embodiment 16

The method of any of the above embodiments, wherein an electricalefficiency for the molten carbonate fuel cell is between about 10% andabout 40% (e.g., between about 10% and about 35%, between about 10% andabout 30%, between about 10% and about 25%, between about 10% and about20%) and a total fuel cell efficiency for the fuel cell of at leastabout 50% (e.g., at least about 55%, at least about 60%, at least about65%, at least about 70%, at least about 75%, or at least about 80%).

Although the present invention has been described in terms of specificembodiments, it is not necessarily so limited. Suitablealterations/modifications for operation under specific conditions shouldbe apparent to those skilled in the art. It is therefore intended thatthe following claims be interpreted as covering all suchalterations/modifications that fall within the true spirit/scope of theinvention.

1. A method for producing electricity, the method comprising:introducing a fuel stream comprising a reformable fuel into an anode ofa molten carbonate fuel cell, an internal reforming element associatedwith the anode, or a combination thereof; introducing a cathode inletstream comprising CO₂ and O₂ into a cathode of the molten carbonate fuelcell; generating electricity within the molten carbonate fuel cell, themolten carbonate fuel cell being operated at a fuel utilization of about60% or less; generating an anode exhaust comprising H₂, CO, and CO₂;separating, from at least a portion of the anode exhaust, a firstH₂-rich gas stream comprising at least about 80 vol % H₂; and combustingat least a portion of the first H₂-rich gas stream to produceelectricity, wherein a fuel utilization in the anode is about 50% orless.
 2. The method of claim 1, further comprising performing a watergas shift process on the anode exhaust, the at least a portion of theanode exhaust, or a combination thereof.
 3. The method of claim 1,further comprising separating CO₂ from the anode exhaust, the at least aportion of the anode exhaust, or a combination thereof.
 4. The method ofclaim 1, further comprising separating H₂O from the anode exhaust, theat least a portion of the anode exhaust, or a combination thereof. 5.The method of claim 1, wherein the separating step comprises: performinga water gas shift process on the anode exhaust or at least a portion ofthe anode exhaust to form a shifted anode exhaust portion; andseparating H₂O and CO₂ from the shifted anode exhaust portion to formthe first H₂-rich gas stream.
 6. The method of claim 1, wherein thefirst H₂-rich gas stream comprises at least about 90 vol % H₂.
 7. Themethod of claim 1, wherein combusting step comprises generating steamfrom heat generated by the combustion, and producing electricity from atleast a portion of the generated steam.
 8. The method of claim 1,wherein the combusting step comprises combusting the at least a portionof the first H₂-rich gas stream in a turbine.
 9. The method of claim 1,wherein the cathode inlet stream comprises exhaust from combustion of acarbon-containing fuel in a combustion turbine.
 10. The method of claim9, wherein the carbon-containing fuel comprises at least 5 vol % ofinert gases.
 11. The method of claim 9, wherein the carbon-containingfuel comprises at least about 10 vol % CO₂.
 12. The method of claim 9,wherein the carbon-containing fuel comprises at least about 10 vol % N₂.13. The method of claim 1, wherein the anode exhaust has a ratio ofH₂:CO of at least about 3.0:1.
 14. The method of claim 8, furthercomprising forming a second H₂-containing stream from the anode exhaust,the at least a portion of the anode exhaust, the first H₂-rich gasstream, or a combination thereof; and recycling at least a portion ofthe second H₂-containing stream to the combustion turbine.
 15. Themethod of claim 1, wherein at least about 90 vol % of the reformablefuel is methane.
 16. The method of claim 1, wherein the molten carbonatefuel cell is operated at a thermal ratio of about 0.25 to about 1.0. 17.The method of claim 1, wherein an amount of the reformable fuelintroduced into the anode, the internal reforming element associatedwith the anode, or the combination thereof, is at least about 75%greater than an amount of hydrogen reacted in the molten carbonate fuelcell to generate electricity.
 18. The method of claim 1, wherein a ratioof net moles of syngas in the anode exhaust to moles of CO₂ in a cathodeexhaust is at least about 2.0:1.
 19. The method of claim 1, wherein aCO₂ utilization in the cathode is at least about 60%.
 20. The method ofclaim 1, wherein the molten carbonate fuel cell is operated to generateelectrical power at a current density of at least about 150 mA/cm² andat least about 40 mW/cm² of waste heat, the method further comprisingperforming an endothermic reaction to maintain a temperaturedifferential between an anode inlet and an anode outlet of about 100° C.or less.
 21. The method of claim 20, wherein performing the endothermicreaction consumes at least about 40% of the waste heat.
 22. The methodof claim 1, wherein an electrical efficiency for the molten carbonatefuel cell is between about 10% and about 40% and a total fuel cellefficiency for the molten carbonate fuel cell is at least about 55%.